An alternate character is a type of character besides the main protagonist that can be controlled by the player.
Often, in games with multiple alternate characters, players get to see multiple sides of the same story by playing through each character's story, as if they're ghosts that can possess multitudes of bodies. Sometimes, each character would have their own strengths, weaknesses, and gameplay styles separate from the other playable characters. Other times, they all play the same as each other with next-to-no differences in playstyle.
Another type of alternate character can be found in Massively multiplayer online role-playing games. Alternate characters in this type of game can be used to carry excess items for the player or allow playing with characters of different races or classes.