While in the lift one day, Arthur Fleck encounters Sophie (Zazie Beetz), a pretty single mum who lives in his apartment building with her daughter. As the story goes on, you see Sophie coming to one of Arthur's comedy shows, spending time with him afterward, and even comforting him while his mom is in the hospital.
Fast forward to when Arthur starts to break down, and it's revealed that he imagined the whole thing — he and Sophie aren't together at all, and never were. In fact, he later breaks into her apartment and when she discovers him on her couch, she forcefully asks him to leave.