Tommy Miller is a secondary antagonist from the sci-fi/thriller film The Butterfly Effect. He is the son of George Miller and younger brother of Kayleigh Miller, Evan Treborn's girlfriend.
We first meet Tommy in a flashback as a young child sitting in the dark corner of his basement with an eerie, emotionless expression on his face while twisting the head off a doll. He tells his father that he wants to be in his "home movie" with his sister, but George tells him that he wants Evan to be in it. George turns out to be a pedophile who takes videos of his children naked.
We cut to a couple years later when Tommy is 12 as he, Kayleigh, Evan, and their friend Lenny sneak into a movie theater playing the movie Se7en. After the movie, Tommy is furious when he sees Evan and Kayleigh kiss (it's implied that he harbors incestuous feelings towards his sister and hates Evan), and he then takes out his anger on a teenager, brutally assaulting him with a pole. As he is then ushered out of the theater, he is shown smiling with sadistic glee.
Later, Tommy kidnaps Evan's dog, ties it in a sack, and pours gasoline on it, intending to burn it alive. As Evan, Kayleigh, and Lenny try to stop him, he viciously attacks them with a piece of plywood and knocks them unconscious before proceeding to burn the dog to death.
In one alternate timeline, we find out that Tommy was institutionalized, but later escaped from the institution as an adult. He then confronts Evan while he and Kayleigh are at college and attacks him with a baseball bat. Evan manages to take the bat from him and eventually beats him to death in self-defense.
Evan travels back in time to try and get Tommy to become a better person. In one timeline, he is successful, but he is left without limbs, as Lenny had been forced by Tommy to plant dynamite in a mailbox which had caused the death of a woman and her baby, and Evan took the dynamite but it exploded and his limbs had to be amputated. In this timeline, Tommy is a devout Christian who devotes his time to helping the less fortunate.
In another timeline, we find out the reason why Tommy is such a psychopath. His father, in addition to molesting him and his sister, beat him on a regular basis. Evan goes back to the moment where Tommy tries to burn his dog to death, but this time, he convinces him not to do it. Unfortunately, Lenny stabs Tommy to death with a shard of metal because he was traumatized by Tommy telling him that he would kill his mother if he didn't plant the dynamite in the mailbox.
In the end, Evan goes back to the day he first met Kayleigh and doesn't befriend her so that she and Tommy won't end up with their father, who is the cause of their problems. Tommy's fate after this is left unknown.