From LEGO Batman Wiki
Harleen Quinzel was a budding psychiatrist with an obsession for the criminally insane, specifically the Joker. While serious business at first, an interview turned into a date after the Joker twisted her mind with possibly false origin stories. After comoing to the conclusion that her "puddin'" didn't belong in Arkham, she donned a jester's costume and became his on-and-off girlfriend, Harley Quinn! She is an accomplished acrobat, who will do anything to prove her love and loyalty to her beloved Joker.
LEGO Batman: The Videogame
After breaking from Arkham, Harley was the Joker's main assistant in his maniacal scheme, fighting Robin with both a hammer and an alligator! After the rest of the team was detained, Harley was the sole member to reach the cathedral with the Joker. However, she was easily taken out after a swarn of bats scared her. She was still obsessing over the Clown Prince of Crime in her cell, seemingly at home.
LEGO Batman 2: DC Super Heroes
In Lego Batman 2 DC Super Heroes, Harley Quinn is first spotted with The Joker in the first mission, along with Joker, Two-Face, Riddler and Penguin. After she and the other villains interrupt the Man of the Year Award ceremony, Batman and Robin confront them. Harley tries to buy the others some time by fighting the duo with her goons, but is defeated and escorted to Arkham Asylum with her allies. Not long afterwards, Lex Luthor and the Joker free her and the other inmates in Gotham City using the Deconstructor. She, along with Two-Face and Riddler, try to escape in Two-Face's truck, but Batman and Robin intervene and the truck is crashed, knocking her, Two-Face, and Riddler unconscious. After the other escaped villains are contained by the duo, Harley and the other inmates manage to apprehend the police arresting them and cause an even bigger outbreak than before. She travels to Amusement Mile, where she is defeated by Batman and Robin.