| Minor spoiler warning!|
The following section contains minor spoilers. You may want to refrain from reading it.
The Minion is actually Red's dead husband. When fed the rune of identity, Minion's revealed name is "@" and he gives Red a mini-quest. @ is the one who left all those bloody messages on the walls and wants vengeance against the Balrog, his murderer. In addition, @ wants to be revived, but there are very few clues left behind to tell the player how to do so.
The minion will run straight into the enemies near you, bashing them with whatever weapon you equipped him. When given a ranged weapon, the minions's priority will be to fire or throw his weapon, even if he is also equipped with a melee weapon. This means that he will back up to a considerable distance to use his ranged weapon, even if the target is near him at first, leaving Red to fight alone against the enemy.
One other use of the minion is to test for cursed items. Since the undead are immune to cursed items, equipping one on him will give you a message, identify the curse, and allow you to unequip the item. This can be done before equipping an item on the player to prevent accidental usage of cursed items. Note that if the minion is polymorphed or wearing a face, he will be unable to take off the item until reverted to skeletal form.
It is possible to resurrect the minion by approaching an Underworld portal. He will rejoin you without any of his equipment, but you are fully able to re-equip him. It is also possible to polymorph the minion or equip a face and cast a rune of undead on him, which will cause him to go back to his normal form when killed. This can be repeated to grant the minion a much longer lifespan.
It is also possible to have a friend play as the minion in Multiplayer mode (accessible through the Option tab on pressing the menu key).