In the film's opening scene, Frank sends three men to pose as Cheyenne's men - and kill a gunman playing a harmonica, who is later given the nickname, "Harmonica". However, Harmonica kills the two men and informs Cheyenne that the gunmen Frank sent were posing as Cheyenne's men. Cheyenne goes on the run when Frank frames him for the murder of the McBain family. Harmonica eventually meets and befriends Cheyenne, and they work together to take down Frank. Harmonica also forces Jill McBain to stay at her home, so he can lure Frank in and kill him himself.
Harmonica arrives on the train where Frank and his employer, Mr. Morton are discussing business, but is caught by Frank who captures him and holds him at gunpoint, before Cheyenne intervenes and they manage to escape.
Later, Frank turns against Morton - and later has sex with Jill, before forcing her to sell Sweetwater at an auction. He tries to scare the other bidders at the auction, so he can pick up the land cheaply for himself.
However, Harmonica arrives - holding Cheyenne at gunpoint - and manages to make a much higher bid ($5,000 precisely), based on the reward money he would received from delivering Cheyenne to the authorities. Despite trying to scare Harmonica, Frank fails to do so - and Jill buys back the farm from Harmonica. After this, some of Frank's men finally turn on Frank himself - having been paid to do so, by Morton.
Harmonica kills them, so he can have the honor of killing Frank himself. With Morton and the rest of Frank's men having been killed in a battle with Cheyenne's gang, Frank goes to Sweetwater and confronts Harmonica. On two occasions, Frank has asked him who he is - but, Harmonica simply refuses to answer. Instead, he names men who Frank has murdered.
Eventually, Harmonica's motive for revenge against Frank is revealed: When Harmonica was younger (a teenager), Frank made his older brother stand on his shoulders, and put a noose around the brother's neck. After his brother kicked him down so that his younger brother wouldn't suffer, his brother was hanged.
Harmonica draws his gun first, and shoots Frank - and, after Frank asks who he is (for a third time), Harmonica puts a harmonica in the dying Frank's mouth. Frank nods (finally recognizing who he is) and dies, avenging his older brother as well as Frank's other victims, and freeing the town of his wrath, proving himself a hero.
He meets up with Jill and Cheyenne and tells them that he plans to leave, Jill hopes he will come back out of gratitude for saving her financially, before he and Cheyenne take off. Cheyenne soon reveals though that Morton shot him during a fight with him on the train before succumbing to his wounds and dying. Harmonica then takes Cheyenne's corpse away as he rides off away from the town.
This unnamed middle aged man was born in america he at least had one brother. When the bandit Frank kidnapped him and his brother, he sadistically hanged his brother on harmonica's shoulders, when he collapsed his brother was hanged, after this he presumably trained with guns and became one of the best gunslingers of the wild west.