What is the name of the Joker's main love interest?

The Joker's main love interest is Harley Quinn. Her original identity is Dr. Harleen Frances Quinzel, a psychiatrist who became fascinated with the Joker while treating him at Arkham Asylum. He manipulated that professional relationship, eventually turning her into his accomplice, partner in crime, and romantic obsession.

Harley Quinn was created by Paul Dini and Bruce Timm for Batman: The Animated Series. She first appeared in the 1992 episode “Joker's Favor” and was initially intended to be a one-episode character. Her popularity led to recurring appearances, and DC later introduced her into comic-book continuity in Batman: Harley Quinn #1 in 1999.

The relationship is famous but deeply abusive and codependent. Stories such as “Mad Love” show the Joker repeatedly exploiting Harley's loyalty and abandoning her when she is no longer useful. Later comics increasingly portray Harley as an independent antihero and explore her romantic relationship with Poison Ivy, but Harley remains the canonical character most strongly associated with the Joker romantically.