A player companion helps the player through the majority of the game. Examples include NPCs npc_alyx and npc_barney. Distinguished from a Player Ally who is a generic expendable character (such as a rebel) and assists the player only briefly.
In some games a Player Companion is invincible; they may be knocked down or play pain animations but cannot actually die. In other cases the Companion has signifiantly more health than normal, but can still die, resulting in a game over.
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