The Alpha Male is the main antagonist of the movie and novel I Am Legend. In the movie he is played by Dash Mihok.
The Alpha Male is one of the zombie-like Darkseekers who have overrun New York after the accidental mutation of the K9 Virus wiped out more than half of humanity and reduced the rest to cannibaistic Darkseekers. Like all the Darkseekers in the film, he is bald, pale, wears rags and ripped clothes, and has bad sense of aggression.
He first met the movie's protagonist Dr. Robert Neville, when his wife and alpha female was taken by Neville to be experimented.
Neville noted the Alpha Male as unusual for the Darkeseekers doesn't show much appreciation to another but the male tries to save his girl which is uncommon for him, and also the aggression showed by the male when he shows himself to direct sunlight, which can kill the Darkseekers.
In the rest of the movie the Alpha Male torments Neville by killing his dog and only companion via unleashing a group of mutated dogs upon the two and forcing Neville to later euthanise his dog to end her suffering. Neville subjected to grief tries to commit suicide by killing a lot of Darkseekers. After Neville crash his car the male tries to take a bite of his face but Neville is save by another survivor.
While they try to escape the Alpha Male follows them and discovers Neville's hiding place. During the night he together with his pack attacks Neville. Both had a showdown where he pushes Neville and the remaining survivors back to Neville's laboratory. Neville sacrifices himself so the other two can live.
In an alternate ending The Alpha Male retained some parts of his humanity. Neville sees that the male only wanted his woman back when he slaps the glass showing a butterfly imprint, the same butterfly on the woman's tattoo. So he puts his gun down and returns the female Darkseeker back to the male. He then apologizes and the male acknowledges it before all of them left.