Happy was an accomplice of the Joker in the robbing of Gotham National Bank.
Happy and Dopey travelled from a neighboring building to the roof of Gotham National Bank with a cable gun. After helping Dopey to disable the bank's alarm system, Happy shot him in the back with his handgun, killing him, then made his way to the safe and started to open it, receiving a jolt of electricity from the security system.
Happy finished opening the safe by wearing his shoes over his hands to turn the lock. Happy, like Dopey, was considered expendable by the Joker after he had done his job, and Grumpy came into the vault and killed him, although not before the latter learned Happy killed Dopey under similar orders.
Happy's body was still laying in the vault when Batman spoke with Lt. James Gordon about the Joker and the mob.
Behind the scenes
Happy took his callsign from one of the Dwarfs in Disney's Snow White and the Seven Dwarfs.
Similar to Dopey, Happy's hair in the movie was red, yet the collector's mask depicting Happy's mask was brown.
Happy killed Dopey with Beretta 92SB-C 9x19mm (compact model of the 92SB).