Speaking to Movieweb, the writer revealed that the follow-up will focus on the characters Hit Girl and Red Mist.
"We left things off with [Hit Girl] being adopted by this extremely nice man. He is a cop. And she has to stop killing people, you know? That seems like an interesting starting point in the next one in regards to her character," he said.
"She is going to be like Clint Eastwood in Unforgiven, where she hasn't picked up a gun in years. It's that type of thing. I like the idea of doing that with an 11-year-old girl."
Millar went on to say that the movie will also see Red Mist attempt to avenge his family's murder. "[Red Mist] is a guy who has had his entire family killed by Hit Girl. He heads off and develops his martial arts skills. He learns how to become a better villain. That type of thing," he added.
"He is going to come back and wreck unholy vengeance on our team. But in reality, he just comes back and gets stoned, and he spends a lot of money. He can't do anything, so he hires this bad ass girl from Russia, she comes into the story, and her plan is to kill Hit Girl.
"Red Mist becomes like Charles Manson. He goes online and influences a bunch of young villains to go against Kick-Ass and Hit Girl. The whole thing ends up as a big gang fight like you'd see in The Warriors."
Directed by Matthew Vaughn, Kick-Ass 2: Balls To The Wall is slated for release in 2012.