She was played by Nicky Whelan.
Regarding Katie's childhood, other than the revelation that her real name was Tara, all that was revealed was that she was placed at foster homes. In her adult life, Tara became an egg donor, but during a procedure, it was revealed that she had an infection in her body that only allowed her three healthy eggs to donate. Tara donated two of her remaining eggs; one to Monica Wheeler and the other to Angela (the film's main protagonist). In Tara's twisted mind, Monica "broke her promise" to make her daughter a priority, and she broke into the Wheeler home and confronted Monica while she was bathing. The enraged villainess bludgeoned Monica to death in the tub, and later took possession of baby Maddie. When she was caught by Monica's husband, Tara grabbed a knife and killed him as well.
After killing the Wheelers, Tara dyed her hair (going from brunette to blonde), took the name Katie Wells, and went to the home of Angela and her husband, Brian, doing so in a mission to reclaim "her" daughter, Cora. She immediately befriended the couple and made such a good impression on them that Angela invited Katie to stay at their home and become their live-in nanny. While doing so, Katie engaged in a sexual relationship with one of Angela's close friends, Linda Morrow, with Angela catching them in the act (while not knowing that Katie was with Linda).
Katie and Linda later met at the beach on the following day, with the latter announcing to her lover that Angela wanted her to become their surrogate (following Angela's miscarriage). Katie acted happy for Linda, but she was actually jealous of her. While Linda was swimming in the lake, Katie (wearing only a bra) went into the water and confronted Linda; revealing that Cora was the result of Katie donating her egg to Angela. She also revealed the truth about Maddie, and stated that she planned to take Cora back after Angela stated that she would be going back to work. Following her revelation, Katie struck Linda with a barbell and held her underwater, killing her.
After Linda's murder, Katie became Angela's surrogate, but as the film progressed, Angela began to notice Katie's true nature, as she referred to herself as Cora's mother on more than one occasion. Katie became hostile towards Angela when she found her holding a pill bottle; as Angela had a past addiction. After learning that Brian and Katie discussed naming the baby, Angela confronted Katie and attempted to inform Brian and Donna (Brian's mother), regarding Katie's nature, only for her accusations to fall on deaf ears.
Despite Angela later apologizing to Katie, the blonde villainess suspected that Angela was getting close to the truth, so she later drugged Angela's drink in an attempt to get her out of the way. Angela feels the effects but survives, and when she returns home from work, she verbally confronted Katie during what was supposed to be a surprise baby shower for Angela. After the guests leave, Katie admitted everything she did in a confrontation with Angela, who was keeping her at bay with a knife. She boasted about how none of Angela's friends believed her regarding her claims, and it was at that moment that Katie took the knife (while still in Angela's possession) and stabbed her own stomach. Katie later bashed her own head on the kitchen table, making it look like she was being attacked by Angela, who later collapsed.
At the hospital, Brian finally learned the truth about Katie/Tara, and then went to Katie and told her that Angela had passed. Shortly afterwards, Katie gave birth to baby Gabriel, and it was at that moment that Brian surprised Katie by bringing Angela out to hold their baby, causing Katie to have a fit. At the film's end, Katie was institutionalized and placed in a padded room.