Katherine McKay is the love interest to Adam Lerner in the film 50/50
Katherine is a psychology student who's interning for her doctorate. At the start of the film, she's only given therapy to 2 previous patients.
When Adam finds out he has cancer, he is recommended Katherine to help him accept his condition. Right away, there is awkwardness between them as Adam is uncomfortable opening up to a young and inexperienced therapist.After being dumped by his girlfriend, Katherine tries to get even closer to Adam as a way of helping him deal with his grief. While driving Adam home one day (his ex-girlfriend used to drive him to his sessions), Katherine tells Adam about her own ex-boyfriend and how obsessed she was. The night before Adam's surgery, he panics and winds up calling Katherine for support. When Katherine tries to make him fell better, Adam says "I wish you were my girlfriend", to which Katherine seems untroubled, considering their professional relationship. On the day of Adam's surgery, Katherine waits with his parents and best friend, reassuring them that he's in safe hands. Adam makes a full recovery and Katherine visits him after the surgery.
Some time later, Katherine visits Adam at his house, implying that they're now dating.