Tracy Stewart is a senior at Beacon Hills High School in Season 5. She is abducted, then experimented upon by the Dread Doctors, becoming a Chimera for their experiment. Tracy has genetic chimerism due to her having an allogeniac skin graft. Her hybrid Chimera traits come from a werewolf and a kanima. Tracy is declared a failure then is executed by the Doctors. Theo brings Tracy back to life, along with Hayden, Corey and Josh Diaz, so to form his Pack with himself as their Alpha. Upon coming back to life, Tracy has been shed of her humanity and becomes Theo's right hand, following his orders and taking to his views unquestioningly, and develops feelings for him. Tracy is betrayed by Theo who steals her power, killing her.
Tracy is at first delicate, shy, reserved and rather faint-hearted being confused by her night terrors, the Dread Doctors' experiments and under stress from her school work. She is a quiet and polite young woman towards her peers and authority figures.
After dying and coming back to life, Tracy has lost her humanity, her morals and scruples. She becomes aggressive, cold, dispassionate and shows little to no emotion, or regard for human welfare, becoming borderline sociopathic. She disaffectedly murders a witness to her supernatural abilities, leaves a trail of bodies in her wake and feels no horror over the idea of having killed her own father. She displays a Wolf's instinct to follow Theo's leadership, being unerringly loyal and affectionate to him.
In Parasomnia, Tracy is in counselling with Natalie Martin, who is acting as the school guidance counselor. She explains that she's been having night terrors and relays one in particular that has to do with a raven knocking at her skylight. In reality her skylight doesn't even open, but in the dream it suddenly opened during a thunderstorm and she got up to close it, balancing on a chair. She saw someone pull the chair out from under her, and in the morning her father found her on the floor.
Natalie Martin tries to convince Tracy that it's just normal nerves. Tracy heads up to leave for class, but she vomits up black ooze all over Natalie's desk, along with a raven feather. Tracy starts having hallucinations while at school. Lydia convinces her mother to let her take Tracy home.
She kills her father and wounds two police officers that are transporting Donovan Donati, then hunts down the other people who have tried to help her, including her therapist and Mrs. Martin. She kills the therapist and tracks Mrs. Martin to the school. Scott, Liam, Stiles, Lydia, and Kira approach her at the school. Tracy grabs Hayden's arm, cutting into her, although the wounds don't stay. She then passes out and leaks silver fluid. They take her to the Animal Clinic for diagnosis.
While there, Tracy comes to and attacks everyone, leaving them paralyzed. She then tracks Mrs. Martin to the Sheriff Station. Lydia and Kira arrive at the station at the same time, since Lydia realized what Tracy's killing pattern was. Kira takes on Tracy cuts off her tail, but not before Tracy slashes Lydia in the side.
Malia recovers from Tracy's venom before Scott and tracks her to the Sheriff Station. She finds Tracy downstairs with Mrs. Martin. Malia is able to take her in the ensuing fight, where Tracy finally wakes up from her night terror. Just as she does, the The Dread Doctors show up and inject her with more silver fluid. They declare her a terminal case and leave her to die.