Kristina Adela Davis was conceived during a one-night stand between her mother, Alexis Davis, and her father, Michael "Sonny" Corinthos Jr. Due to the fear of Sonny's lifestyle as a known mob boss, Alexis moved heaven and earth to prevent the true paternity of her daughter from becoming public knowledge. When Alexis' ex-fiancé, Ned Ashton, discovered she was pregnant with Sonny's child, he proclaimed himself the father of the baby to protect it. When Alexis gave birth to a baby girl, she named the infant after her recently killed sister, Kristina Cassadine.

Alexis did everything she could to both protect her daughter and keep her daughter close. For a while, though, Kristina was placed in the Quartermaines' custody while her mother sought mental help for killing Luis Alcazar, who had been responsible for Alexis' sister's death. Then Kristina's "cousins," A.J. and Skye Quartermaine, kidnapped the girl; she was reunited with her mother against court orders, when Alexis kidnapped her from A.J. and Skye. Alexis eventually returned Kristina to the Quartermaines but dressed up in drag as a butler to be near Kristina. It took months for Kristina to finally be given back to her mother legally, at which time she received a proper, secure home.

In November of 2004 Kristina was diagnosed with a form of leukemia that required a bone marrow transplant. Dr. Steven Webber concluded that Kristina's best bet of a donor was from a relative, but after testing Ned Ashton, he discovered that Ned could not be Kristina's biological father. As Kristina lay in the hospital, her mother was forced to reveal Kristina's paternity to all who mattered, including Sonny. After Sonny's family members proved not a match for Kristina, another option was given, which was to harvest the stem cells from Sonny's unborn daughter with Sam McCall by inducing labor two weeks earlier than the due date. Sam refused at first, but Alexis pleaded and pushed for Sam to save Kristina, which eventually forced Sam into labor and saved Kristina's life at the price of Sam's baby girl dying at birth.

After Kristina's recovery from leukemia, Sonny and Alexis battled it out in court for custody. Alexis had married Kristina's half-uncle, Ric Lansing, to better her chances at wining custody of her daughter. After many ups and downs in court, Alexis agreed to a shared custody agreement with Sonny. Alexis still feared Sonny's lifestyle would eventually hurt Kristina and his other children. Alexis was not mistaken because right after that, Kristina was kidnapped.

At first Alexis was suspected, but shortly afterwards, Kristina's brothers, Michael and Morgan, were kidnapped as well. It was revealed that Faith Roscoe, along with Michael's biological father, A.J. Quartermaine, had orchestrated the whole thing to get back at Sonny. Kristina was eventually found and returned home where she learned the news that she would be getting a little sister. After the birth of her little sister/cousin, Molly Lansing, Kristina soon welcomed a big sister into her life when it was revealed that Samantha McCall was her half-sister through her mother, Alexis.

While still a young girl, Kristina walked into an ambush meant for her sister Sam McCall, Jason Morgan, and Damien Spinelli that was orchestrated by Lorenzo Alcazar, Luis Alcazar's brother. During the ambush, Kristina witnessed Sam shoot and "kill" Lorenzo's son, Diego Alcazar. Kristina went into complete shock and remained in a silent trance for weeks until finally speaking to wish her mother a "Merry Christmas." She then returned to her silence after seeing the presumed-dead Diego Alcazar alive as the Text Message Killer.

Kristina was heard about but not seen until June 2009, when she appeared as a 16-year-old who was caught underage drinking by her father's mob enforcer and best friend, Jason Morgan. Kristina as a teenager had become very manipulative and rebellious while still maintaining the picture of perfection to her mother. Kristina's feelings toward her father, Sonny, were of great disappointment that he hadn't tried hard enough to reach out to her or see her. When her brother Michael awoke from a year-long coma, she began to reconnect with her adoptive brother by showing him what he had missed. Kristina introduced Michael to her older boyfriend, Kiefer Bauer, which ended in a bar fight at Jake's.

As Michael and Kristina got closer, Alexis worried that Michael was not a good influence for Kristina due to his brain being slightly damaged due to the gunshot wound and the surgery, and Alexis asked that they limit their time together, especially after she found them at Jake's. Kristina still continued to see her brother when Michael moved in with Sonny. During a visit, she found a wounded man in her stepmother Claudia Zacchara's room. He revealed himself to be Dominic Pirelli. Unbeknownst to Kristina, Claudia had hired Dominic to kill Jason Morgan but he had failed.

Kristina agreed to help Dominic escape, but Claudia found them and told Kristina to get home before Sonny and Alexis found her there. Later on, when visiting Alexis at the Port Charles Police Station, Kristina witnessed Mayor Floyd confess to having an affair with her mother, Alexis Davis. He further claimed that Alexis was framing him for the murder of his other mistress, Brianna Hughes, because he had dumped Alexis. The scandal affected both Kristina and her sister Molly's school life. It got even worse when Alexis herself was considered a suspect when e-mails to the mayor surfaced that painted Alexis as an obsessive ex-lover capable of murder.

When Michael began living with the Quartermaines Kristina began to visit her brother again and continued showing him the things he had missed out on. After another fight between Kiefer and Michael at the country club, Kristina took Kiefer back to her house, where Kiefer slapped Kristina after she refused to have sex with him. Kiefer apologized but continued trying to have sex with Kristina, who again refused, which made Kiefer mad and he left. Her sister little sister Molly was the first to discover the bruise on Kristina's cheek, but Kristina said she had just been clumsy. Afterwards, she drove back to the country club, only to be disappointed to see Kiefer with another girl. After confronting the two, she left very distraught. While driving, she got distracted and drove in the wrong lane and believed she had caused another vehicle to go off the road. She called Kiefer who had a change of heart and helped Kristina cover up what had happened.

Kristina's guilt began to eat at her when it was revealed that her stepmother, Claudia Zacchara, had been in a car accident on the same road Kristina had been on. Then her brother Michael confessed to causing the accident due to road rage. Later, when Kristina sneaked into the Quartermaine Mansion, she overheard her father mad with Michael and telling him that he had killed their baby brother that Claudia was pregnant with. Michael's guilt and regret led him to run away, and Kristina felt she was even more responsible and went with him to Mexico. Meanwhile Sam's suspicion of Kristina causing the accident grew stronger after Kristina took off with Michael and forensics placed one of Alexis' cars at the scene of the accident.

While Sam and Jason went after the teens, Alexis confessed to causing the accident as a way to protect her daughter. Sam and Jason weren't the only ones after the teens; Claudia had hired Alexis' ex-boyfriend and orchestrator of the Metro Court hostage crisis, Jerry Jacks, to find Michael and Kristina. Kristina and Michael bumped into Jerry in Vera Cruz, Mexico, after encountering trouble with some goons. Afterwards, Jerry gave the two a nice hotel room to stay in while he secretly planned a trap for Sam and Jason.

Michael and Kristina got away from Jerry's hotel room and fled to Cancun, Mexico, as planned. They worked at a store on the beach. Michael found a discarded cell phone, and they both used it to send a message to their parents, saying they were okay. Kristina began to feel homesick, especially for Kiefer, and called him, but another woman answered and hung up saying Kiefer was busy. Kristina was worried she had lost Kiefer and wanted to return home, but before she could, she revealed the truth to Michael that she might have caused Claudia's accident.

Sam and Jason returned Michael and Kristina to Port Charles. Kristina felt guilty when she learned her mother had confessed to the hit-and-run to protect her. Sam encouraged Kristina to take responsibility for what she had done, but Kristina went along with the decision her mother had made to cover for her. Claudia overheard Sonny and Alexis discuss Kristina's role in the accident, and she vowed to make Kristina pay. When Kristina apologized to Claudia for causing the accident, Claudia pretended to forgive her.

Sonny arranged for all his children and Molly to attend the hospital carnival together. Kristina received a warning from a fortune-teller that danger lurked nearby. Kristina tried to sneak away from her family to spend some time with Kiefer. Claudia wanted to talk to Kristina privately about her boyfriend and convinced her to go on a ride. While they were on the ride, Edward Quartermaine drove his car into the carnival. Kristina was trapped underneath the ride until Michael rescued her. Kristina thanked him and confided in Michael that Claudia had walked past Kristina several times while she was trapped but had not seen or heard her.

The relationship between father and daughter remained tumultuous for Kristina and Sonny. She was jealous of the bond Michael and Morgan shared with Sonny because he had been a constant part of their lives. Although Alexis had kept Sonny at bay while Kristina grew up, Kristina felt Sonny did not try hard enough to have a relationship with her and never had time for her. When Kristina learned Sonny was considering having another child with Claudia, she recommended he not bring another child into the world, since he could not be there for all the children he already had.

As Kristina continued to navigate through her feelings toward her father, she also worried she was going to lose Kiefer if she did not have sex with him. Kristina convinced Sam to take her to a clinic, and she obtained birth control pills. Kiefer took Kristina to a motel, and they had sex. Afterward, Kristina regretted her decision.

Just as Kristina started to bond with Sonny, she witnessed his public verbal attack on Claudia. She was shocked by the anger and coldness her father possessed. Kristina tried to talk to Kiefer about her father, but he was more interested in pressuring Kristina for sex. Kiefer struck Kristina after Sonny's bodyguards paid him a visit. The bodyguards warned Kiefer to improve how he treated Michael. Kiefer criticized Sonny to Kristina and blamed her for the threat the bodyguards had issued to him. Kiefer continued to abuse Kristina, and he was jealous of Kristina's friendship with Ethan Lovett. Kristina tried to make excuses to avoid being alone with Kiefer, but her sister and mother continued to push them together.

Kristina believed her father had killed Claudia after she was reported missing. Kristina slammed Sonny to Michael, which prompted Michael to admit that he, not Sonny, had actually killed Claudia. Kristina was shocked by the news. As she tried to process what Michael had done, she pushed Kiefer away. Kiefer responded with physical aggression.

Kiefer and Kristina broke up temporarily, and she pursued Ethan. He was older and did not reciprocate her romantic feelings. Kiefer and Kristina got back together. Kiefer became extremely violent with Kristina after she stood him up to make one more attempt at dating Ethan. Kiefer followed Kristina home to an empty house, and he attacked her. Ethan found her and took her to the hospital. Kristina named Ethan as her attacker but refused to file charges against him.

Kiefer was relieved Kristina had protected him, although Michael, Jason, Lucky, and Dante suspected Kiefer had abused her. Sonny threatened to make Ethan pay for hurting his daughter until Alexis convinced him violence would be detrimental to Kristina because she would blame herself for Sonny's actions. Kristina's fears regarding Sonny's retaliation were more prominent when she heard Claudia's brother testify at Claudia's murder trial that Sonny was abusive to Claudia.

When Kristina told Kiefer she was going to tell the truth about what he had done to her, he beat her and left her unconscious. Alexis found Kristina and rushed her to the hospital. On the way to the hospital, Alexis unknowingly ran over Kiefer. He died as a result of his injuries.

Kristina admitted that Kiefer was her abuser. Kiefer's parents were cruel to Kristina. They accused her of lying and blamed her for Kiefer's death. Kristina was released from the hospital, but she struggled with her feelings regarding Kiefer's death and the abuse she had suffered. The Bauers vowed to prove Kristina had lied and that Ethan was her attacker. Kristina also worried about the legal consequences the Bauers threatened against Alexis.

In order to help Kristina deal with the abuse from Kiefer, she started therapy. The therapy generated several questions Kristina had regarding her relationship with her father and his treatment of women. He attended a therapy session with Kristina, but it did not provide the answers she sought. Instead, Kristina turned to Claudia's brother, Johnny, to learn about Sonny's treatment of Claudia. Johnny told Kristina his sister's life was destroyed by Sonny's verbal abuse, and Johnny admitted he blamed Sonny, not Michael, for Claudia's death because Sonny had driven Claudia to become desperate and unstable.

Kristina wanted to punish her father for being abusive to women and for exposing Michael to a life of violence. She convinced Johnny to pretend to date her. Kristina knew her father would be bothered when he saw she was dating an older man, especially one of her father's enemies. The charade with Johnny went on for a few weeks until Johnny decided to put an end it to, since they had achieved their desired results. They were both unaware Sonny had planted a car bomb in Johnny's car. When Johnny walked Kristina to his car, he sensed something was off and protected Kristina when the car exploded. Neither was hurt physically, although Kristina was shocked her father had planned to kill Johnny.

The threats both Kristina and Alexis received from the Bauer family ended in a dramatic way. Warren Bauer went after Kristina at the hospital and held a gun on her. He blamed Kristina for Kiefer's death and accused her of lying about the abuse. Michael pushed her out of the way, while Mac Scorpio and Ethan tried to contain Warren. Mac and Ethan were both injured and recovered, but Warren was killed in the altercation.

The rollercoaster relationship between Sonny and Kristina continued. They bonded when they discussed Sonny's past relationship with Brenda Barrett. Sonny explained how he understood what it felt like to be abused, and he told Kristina about the abuse he suffered as a child at the hands of his stepfather. Kristina appreciated his honesty and support. Shortly after, Sonny was charged with the attempted murder of Johnny Zacchara, and Sonny jumped bail, although he returned soon after.

Kristina was involved in a bus crash when she was headed on a ski trip. Although she developed hypothermia, she fared much better than her friend Ali. Her friend suffered a head injury and died. Kristina was devastated by the loss of her friend, but she was cheered up a bit when she was asked to be a bridesmaid in her father's wedding to Brenda Barrett. Kristina asked Ethan to be her date to the wedding, since his wife was out of town. Ethan took care of Molly and Kristina when chaos erupted at the wedding. Sam was injured in a car bomb, and Brenda was kidnapped. Ethan's calm and caring demeanor impressed Kristina so much that she openly told Ethan she planned to marry him one day.

As high school graduation drew near, Kristina worried because she had not yet been accepted into Yale, her dream school. A doctor named Lisa Niles sympathized with Kristina and shared her own story of being wait-listed for college. Kristina also started babysitting for Patrick and Robin. Kristina was unaware Lisa had befriended her to gain access to Robin and Patrick's home. Lisa gave Kristina "herbal supplements" to help reduce her stress and she visited Kristina while she babysat. When Kristina offered Ethan one of her pills, he identified them as narcotics. Ethan helped her break her dependency on the pills.

Kristina sought advice from her stepmother, Brenda, about how to dress more maturely and attract an older man. Much to the dismay of her parents, Kristina changed her look. At the same time, Ethan learned his wife had cheated on him and was leaving him for her new lover. Ethan decided to get a quick divorce in the Dominican Republic. With Molly's help, Kristina surprised Ethan on his trip and helped him navigate his divorce. Alexis and Sonny mistakenly believed Kristina and Ethan had run off to be married and followed them to the Dominican Republic. When Kristina returned home, her parents made it clear she was too young for Ethan, but she refused to listen.

The day of Kristina and Michael's high school graduation arrived, and Alexis hosted a party for the family. Kristina continued her pursuit of Ethan, although he remained adamant they would not be more than friends. When Ethan stepped in to protect Kristina from Anthony Zacchara's threats, Kristina took it as a sign that Ethan felt the same romantic interests. Despite Ethan's protests that Kristina was too young, she continued to pursue him.

Ethan flirted with Abby in Kristina's presence and hinted that something had started between them when Abby had been an exotic dancer at Johnny Zacchara's club. Kristina refused to fall for Ethan's act with Abby so he tried another tactic. He agreed to spend time with Kristina but only on his terms. He invited Kristina to join him for an overnight trip to Las Vegas. Kristina made plans to join Ethan on the trip until she overheard him talking about her with Skye Chandler. Ethan admitted he felt sorry for Kristina and had befriended her because she seemed like a nice kid and he had hoped the idea of a trip to Vegas would have scared her away.

Kristina was crushed by the news that Ethan was not romantically interested in her and she went to the pier and threw away the friendship bracelet Ethan had given her. While she was there, she experienced neck pain similar to the pain she had been experiencing periodically for the past few weeks. Kristina passed out and was found by Anthony Zacchara. He called an ambulance and Kristina was admitted to the hospital.

Patrick Drake delivered the news to Kristina that her recent bouts of dizziness and neck pain were caused by a bone fragment that was placing pressure on her spinal cord and she would require surgery. The surgery held a risk of leaving Kristina permanently paralyzed but Kristina agreed to the surgery. Ethan visited Kristina in the hospital and promised to dance with her at her sister's wedding when she recovered and Kristina used that as her motivation. As she awaited the surgery, her family distracted her with discussions about the plans for Sam's wedding to Jason instead of the cause of the bone fragment.

Everyone initially assumed the bone fragment had been caused by the abuse Kristina had suffered from Kiefer until Molly hypothesized that Sonny might had inadvertently caused the damage when he planted the bomb in Johnny's car. Sonny's immediate reaction was to go after Johnny and blame him. Kristina was furious with her father and did not want to see him, although he disregarded her wishes and remained with her until she was taken into the operating room.

Kristina's surgery was performed by Matt Hunter instead of Patrick and Matt revealed that a complication had arisen during the surgery. While everyone waited for Kristina to wake up after her surgery, Johnny sneaked into her room for a visit. He apologized for ever involving her in his vendetta against Sonny and acknowledged his share of the blame if the bone fragment had been caused by the car bomb. Kristina woke up as Sonny entered and accused Johnny of manipulating Kristina. She sided with Johnny and retorted that he took responsibility for his actions instead of blaming others.

After the surgery, Kristina experienced temporary paralysis in her leg but she recovered in time to attend her sister's wedding reception. At the reception, she spoke to Kate Howard about the possibility of working for her so she could pursue a career in fashion. Kristina's plans changed abruptly when she was notified she had been accepted into Yale.

Kristina packed for college and attended a going-away party in her honor hosted by her father. She shared heartfelt goodbyes with her family and she thanked Sonny for never giving up on her regardless of the number of times she tried to push him away. Ethan also stopped by the party and presented Kristina with another friendship bracelet and he encouraged her to enjoy college life. She agreed she would try but promised her heart would be his when he was ready. Kristina left for college immediately after the party.