After boarding school, Kate Valentine, daughter of Katherine Chancellor's long time maid and companion, Esther Valentine, legally changed her name to Chloe Mitchell and was living in New York City. Chloe had an ongoing affair with Billy Abbott. But after Chloe discovered that he had cheated on her, it broke her heart, and she returned to Genoa City pregnant. Chloe tricked Cane Ashby into believing her baby was his and into marrying her. But Chloe went into labor during a snow storm, and stopped at a cabin where Billy was on a date with Lily Winters. Billy had to turn and deliver his own daughter while being talked through it by a doctor on a cell phone. The baby was fine, but Chloe was hemorrhaging and passing out, so they constructed a litter and carried her and the baby down to the waiting ambulance.
The next day, Chloe confessed to Cane that her baby was not his, and Billy verified that he must be the reluctant father. Cane refused to give up the baby and signed the birth certificate naming himself the father and naming the baby Cordelia Katherine Valentine Ashby. Then Cane filed for divorce and sole custody of Cordelia, and Chloe and Cordelia moved into the Chancellor estate with her mother Esther and Billy's mother Jill. Billy married Chloe to be able to fight Cane for custody of Cordelia. Just prior to the hearing, Cane dropped the custody suit so Cordelia would never grow up without a mother like he had, and Cordelia's last name was changed to Abbott. But although Billy grew to love Cordelia and doted on her, he had a hard time settling down to one woman, and he and Chloe eventually divorced.
Victoria Newman became the person to save Billy from a downward spiral of alcoholism, womanizing, and a gambling addiction, and they were married. They bought a house with room for Delia and Victoria's son from a prior marriage, Reed, during their visitations. Unable to have children of their own, Billy and Victoria adopted a baby girl, and named her Lucy. Billy hired Jana Hawkes, the estranged psychopathic wife of Kevin Fisher, to move into their garage apartment and become Lucy's nanny. Unknown to Victoria, Lucy was a black market baby, and Jana had discovered her real identity was the daughter of Daniel Romalotti and Daisy Carter. When the baby broker threatened Jana, she disappeared with Lucy and Delia. Alerts went out that the children were kidnapped, and all the Newmans and Abbotts joined the police search for Jana and the children. Chloe tore into Billy about the many times she had warned him about Jana's instability and danger to their children.
Jana called Kevin to meet her and when he arrived, Jana handed Lucy to Kevin and informed him that she was his half-sister Daisy's baby. But delusional Jana told Kevin that Lucy belonged with them, that they could reunite, leave town, and become a family together. Kevin convinced Jana to leave the children at a church, and Kevin text-messaged Billy with their location. Chloe, Victoria and Billy arrived with the police and reclaimed their children. Once home and safe, Billy confessed to Victoria that Lucy was bought on the black market, and that Daniel and Daisy were her real parents. Although Daniel wanted Billy and Victoria to legally adopt Lucy, the courts awarded custody to her mother, Daisy. Billy and Victoria split up over it, and Billy went back to drinking and gambling and lived in a beat up mobile home.
Chloe and Billy's testimony about Billy's bad habits and run-ins with the law left Billy without a prayer of keeping joint custody of Delia. Then Victoria surprised everyone by testifying on Billy's behalf about what a devoted father he was. Billy tried to thank Victoria afterward, but she let him know that nothing had changed between them. Late that night Billy showed up at the Chancellor estate to see Delia, but Chloe refused and sent him away. Billy got drunk, wrecked his car, and ended up in the hospital. Victoria later learned that he had left for Hong Kong. Weeks later, Chloe received a Jade bracelet for Delia from Billy.
Not long after Chloe and Kevin became lovers, Delia was diagnosed with acute myelogenous leukemia, and needed a bone marrow transplant to recover. Everyone in Genoa City was tested with no match, and Victoria and Billy's brother Jack were determine to find Billy, hoping that he could save Delia's life. Hearing that Billy had left Myanmar, a country in Southeast Asia, Jack left town to find him. Meanwhile, Victoria's father, Victor, had found Billy in a jail cell in Myanmar, and made a deal with him to negotiate his freedom if he promised to never see Victoria again. Billy told Victor to go to hell. Victor told him about Delia's condition, and that Billy might save his daughter's life if he were a match. Victor returned Billy home to his trailer and coerced Kevin to agree to pose as the donor of Billy's blood sample. Billy snuck into the hospital unrecognized, wearing a hooded sweatshirt, to see Delia. Delia told Chloe that her daddy had been there, but everyone assumed it had been a dream. The blood sample proved to be a match for Delia, so Victor arranged for a doctor to harvest the bone marrow from Billy and led everyone to believe that it had come from Kevin.
Meanwhile, Victoria continued to believe in Billy, and emailed him that she loved him and would forgive him if he would just come home. Billy received Victoria's emails on his laptop at his trailer, and wanted to go home to Victoria. But Victor kept him in hiding by threatening to reveal Billy's crime that had gotten him thrown into prison. Billy poured out the booze that Victor brought him, and he told Victor that Delia needed him, so he wanted to keep himself the best that he could be to save her life.
After Delia had chemotherapy, Victor arranged with a doctor for Billy to donate the bone marrow for her transplant, and they prepped Kevin to know how to act as if he had been the donor. The transplant was successful, Delia was expected to make a full recovery, and Victor arranged for Billy to disappear from their lives in a few days. Billy led Victor to believe that he had left town, but instead he holed up in a room somewhere. Billy showed up at the hospital dressed as Santa Claus to see Delia. Delia did not recognize him, and he enjoyed some time with her. Billy eventually got help in clearing his name in Myanmar and was able to escape Victor's control and reunite with Victoria. And Delia went into remission. Chloe married Kevin and the two couples had an amicable relationship due to their shared love of Delia.
In the fall of 2013, seven-year-old Delia was cast as the wicked witch of the west in the school play of The Wizard of Oz. Billy and Chloe enjoyed helping Delia rehearse her lines, and she was a natural little 'ham'. Billy even surprised them both with a dog in the likeness of "Toto" to enhance her part. Due to its love to escape, the dog was named Dash, and went home to live with Billy and Victoria. The night of the play came, and Delia was a hit, getting a standing ovation from the crowd. Afterward, Chloe and Kevin caught a late movie, and Victoria waited at home with a party for the arrival of Billy, Delia, and Dash. Billy stopped to pick up ice cream, leaving Delia and Dash in the car because she would not leave her dog there alone. But when Delia let Dash out of the car to relieve himself, he ran across the road. Delia ran after him, and was stuck by a car. Billy found the car door open and discovered Delia beside the road. During the five minute wait for the ambulance, Billy visualized the welcome home party that was supposed to be, being the protective father before Delia's first date as a prankster teenager, her college graduation party after getting her business degree with honors, Delia's choosing to work with Billy at his renamed club "Dad and Delia", Delia announcing her engagement and looking forward to a family life like Billy had, walking her down the aisle at her wedding, dancing at the reception, Delia's first baby, a son she named William, then their painful parting as she and her family were moving to California. Although she had not spoken or moved, Delia was still alive when ambulance arrived, but she died from massive injuries later at the hospital.
Billy blamed himself for leaving Delia alone in the car, as did Chloe who was inconsolable, refusing to leave the trauma room where Delia had died. She lashed out when approached about organ donation, then finally was allowed to see her daughter's body. Jack found Billy sitting on the side of the road at the accident site and took Billy home. Victoria later found Billy watching videos of Delia over and over. Once Chloe found out that her best friend Chelsea's baby, Connor, needed a cornea transplant, she agreed to donate Delia's. The transplant was successful and Connor was able to see thanks to Delia.
The day of the funeral, Chloe and Billy united to honor their daughter, while Adam stood outside the chapel door, guilt-ridden. The Abbott, Newman, Chancellor, and Winters families attended. Chloe read the poem that Delia had written for Katherine's funeral "Being Free", and she placed Pinkerton Pony with Delia so that he could watch over her as she slept. Delia was eulogized as a "beautiful bubble of giggles and joy." Afterward Jill and Esther held a tea party at the Chancellor estate in Delia's honor where the guests all wore red boas and fancy hats, and a cup of tea was placed for Delia as well. Later Adam wept as he visited the roadside shrine of photos, candles, cards, a Wizard of Oz program, balloons, stuffed animals, and flowers; and Adam left white a rose. Days later, her dog "Dash" showed up at home. Billy welcomed him, but could not bear the reminder, so Victoria found him a new home.
Billy and Victoria attended the ceremony held by the school for the one year anniversary of Delia's death. Her former teacher handed Billy a paper Delia had written called "Lots of Stuff I know for Sure" which was very insightful. Later Billy saw Delia, and said that he wanted to be the kind of man that she believed he could be. Delia said that she had come back to help him, suggesting he stay close to his family who were there to help, and disappeared saying "Happy thoughts, Daddy." Chloe and Kevin visited the roadside memorial, and later viewers saw Adam place a rose there. Jack announced a new Delia project enhancement, a children's theater, with final donations courtesy of Victor and Nikki.
On Valentines Day 2015, Billy visited Delia's memorial on what would have been her birthday, leaving her a copy of the plaque that had been placed on the children's theater which had been built in her memory. Delia appeared to Billy, and asked him if he had forgiven Adam yet. Billy told her that was something he had not done, but would work on it for her.