Jonny Frost was first seen holding down Harleen Quinzel while the Joker electrocuted her. After this, he accompanied the Joker and Harley Quinn to a strip club for a meeting with arms dealer, Monster T.
He later informed the Joker of Harley's whereabouts in Belle Reve prison. He was present while the Joker captured Captain Griggs and attacked Van Criss' facility to find out how to disable Harley's nanobomb.
Frost is shown to be incredibly loyal to the Joker. He carries out orders for his deranged boss without question, attacking the defenseless psychiatrist Harleen Quinzel and countless scientists. He doesn't show any concern for human life; he is willing to murder anyone that his boss wants dead. He watches his boss' meeting with Monster T with great curiosity, interested to see its outcome. He warranted no reaction when the latter was killed. He has a dark sense of humor, as evidenced by his sarcastic and snarky comments during Grigg's interrogation.
- Before his role was revealed, Parrack was rumoured to be playing Slade Wilson aka Deathstroke.
- Jonny Frost is a main character in the 2008 comic Joker. He serves as the narrator throughout the comic, which ends in his death.