|First appearance||Get Outta My Dreams, Get Into My Mouth|
|Last appearance||House Hunting|
She comes to drop off a rent check with Aidan alone in the house, and his lack of blood and Beth and Holly's taunting cause him to come off as odd. Kat later goes back to the house while Erin is in the hospital, and Aidan helps her fix her plumbing, while also revealing her interest in early American history, to which Aidan is well acquainted. The two hit it off, and later he asks her out on a date, and she agrees. Aidan tries to break it off with her, for fear of retaliation from Nora, but he continues to see her until he is approaced and imprisoned by Liam. Kat tries to find out what has happened, prompting Josh's rescue, and after Aidan is infected, Nora invites her over so they can see each other one last time.
After Aidan is cured, he approaches her at the university and passionately kisses her, while remembering his wife's words to him centuries before that he should seek out new love when he finds it. After a disastrous date at the Boston Rare Book Archives where Kat comes across her ex-boyfriend Jeff Westin and his new date Marissa, a college student who seems to be in the same situation Kat was in years ago, they reveal their plans for the future, in which Kat wants to have a family of her own, even though Aidan cannot provide this for her. They start to get intimate, only for Aidan to back away when the bloodlust overcomes him. Outside, after drinking from a blood bag he has with him, goes back to Kat and they sleep together. However, Aidan awakes in an alley hours later, unknowingly having killed and turned Jeff Westin. Kat is invited for Josh and Nora's wedding, where she is upset that Aidan left her alone after their first night together, but Aidan later apologizes by saying it had been a long time since he had last been intimate with someone and got scared, but promised that it would not happen again and they end up on good terms, with Aidan later admitting that Kat is his girlfriend.
Kat later participates in the wedding, being one of Nora's bridesmaids alongside Emily and a spectral Sally.
Kat and Aidan's relationship continues throughout season 4, with things progressing and becoming far more serious. Kat decides to lay her true feelings for Aidan on the line by telling him that she loves him. Aidan appears slightly offput by this statement, and doesn't reciprocate. As time goes on however, Aidan comes to terms with his feelings for Kat, prompting him to give her a speech about their relationship to which Kat believes he is breaking things off with her. Once Aidan senses Kat's worry, he quickly puts it to rest by assuring her that he likes their "routine" and tells her that he loves her. After a serious talk with his now vampiric wife from his mortal life about how vampire-human relationships always end, Aidan decides that this is not what he wants for Kat. This prompts Aidan to tell Kat the truth about who, or rather what, he really is. After showing up at Kats front door, he tells her that he can't go any further with their relationship untill he shows kat his true self. He attempts to tell her straight up that he is a vampire, but she responds with histerical laughter, thinking it all a joke. After Aidan reassures her that he is in fact a vampire, Kat begins to believe Aidan might be mentally ill, and vows to get him help. Slightly frustrated, Aidan pulls her head towards his chest and asks her what she hears. Taken back by his lack of a heartbeat, Kat begins to get flustered. Aidan then tells her that he is going to show her something, but she needs to trust him. After giving Aidan a slight nod, he reveals his black eyes and Kat is sent into a terrified panic. After a short talk in which Aidan reveals that Josh and Nora both know he is a vampire, Kat tells Aidan that she is afraid of him, to which Aidan offers to leave. Kat tells him that he should, and Aidan walks out of the house with a clear look of regret on his face. Aidan spends the next few days wallowing in self pity, and asks Nora if she has heard from Kat. Nora responds by telling Aidan that she hasn't heard from her, and that she is pretty sure Kat broke up with all of them.