Super

Short-order cook Frank Darbo recalls his only two good memories from a disappointing life: marrying his wife, Sarah, and an incident in which he directed a police officer to catch a purse snatcher. Frank immortalizes these two events in a pair of crayon drawings that he hangs on his wall for inspiration. Sarah, a recovering addict, leaves Frank for Jacques, a charismatic strip club owner who gets her hooked on drugs. Frank sinks into depression, where he has a vision in which he is touched by the finger of God and meets the Holy Avenger, a superhero from a public-access television show, who tells Frank that God has chosen him for a very special purpose. Frank believes that God has chosen him to become a superhero and goes to a local comic book store for inspiration. His claim that he is designing a new superhero is met with enthusiastic appreciation from the store clerk, Libby. Frank creates a superhero costume and assumes the identity of "The Crimson Bolt."
Armed with a pipe wrench, he begins to fight crime by delivering savage beatings to various rulebreakers, ranging from drug dealers and child molesters to a man who cuts in line at the movies. The Crimson Bolt soon becomes a media sensation. Initially, the media view him as a violent psychopath, but he begins to gain public appreciation after the criminal backgrounds of many of his victims come to light. Frank later attempts to rescue Sarah, but Jacques' thugs recognize him under the costume and shoot him in the leg as he flees.
A wounded Frank goes to Libby for help. Libby cajoles Frank into letting her become the Crimson Bolt's "kid sidekick," christening herself "Boltie" and designing a costume. She proves to be even more unhinged than Frank, using her superhero guise to nearly kill a man who possibly vandalized her friend's car. Frank decides to let her go, but changes his mind when Libby rescues him from some of Jacques' thugs at a gas station. Libby soon becomes enamored with Frank, but her advances are turned down as Frank insists that he is still married. Arguing that it is different when they are in their superhero identities, Libby rapes Frank while the two are in costume. Frank runs to the bathroom and vomits, where he encounters a vision of Sarah in the toilet. He decides that it is time to rescue her from Jacques.
Armed with guns, pipe bombs, and bulletproof vests, Frank and Libby sneak into Jacques' ranch, killing the first few guards they encounter. However, they are both shot. Frank is struck in the chest, his bulletproof vest sparing him, but Libby is shot in the head and killed. Devastated by her death, Frank goes into a rage, slaughtering all of Jacques' thugs. Inside, Jacques shoots Frank, but Frank gains the upper hand and stabs Jacques to death as Sarah watches, horrified.
Frank takes her home, and she stays for a few months before leaving him again. This time, however, she manages to overcome her addiction and uses her experiences to help others with similar problems. She remarries and has four children. Frank is convinced that her children will change the world for the better. Frank, now with a pet bunny, looks on his wall of happy memories. The wall is covered with pictures of his experiences from his time spent with Libby and pictures of Sarah's kids, who call him "Uncle Frank." Frank looks at Libby's picture, and a tear runs down his cheek.

Question: What does Libby cajole Frank into letting her design?
Answer: a costume