The player is, of course, a person playing the game, either alone, or as one of several in multiplayer mode. Each player is also an entity in the game world, having attributes such as position on the map, health, and items possessed.
Since the game is in "first person", a player does not see himself or herself on the main portion of the screen, except for his or her own hand when certain weapons are selected.
In Doom, a marine's face is shown on the status bar, where it reflects the player's health (becoming progressively bloodier as the player's health falls) and reacts to events such as being attacked, stepping on a damaging floor, or finding a weapon or powerup.
The player moves in response to the keyboard, mouse and/or joystick at either walking speed or running speed. The forward/backward speed is 25 or 50 map units per game tic; the sideways speed is 24 or 40 (or, in some cases, 50). These movements can be combined: see straferunning.
In a multiplayer game, the other players appear as space marines in differently colored space suits. In vanilla Doom, the colors are green, indigo, brown and red. Source ports typically permit more choices of color, and sometimes more than four players. The sprites for a player have the prefix PLAY, and are rendered in a green suit. To draw another player, the Doom rendering engine remaps the shades of green to shades of the desired color.
A player spawns with 100% health, a pistol, and 50 bullets. The fists may also be used as a weapon. When respawning in vanilla Doom, everything else (armor, other weapons, ammo, powerups, and keys) is lost. In deathmatch mode, an exception is made to this as players spawn with all keys. Some source ports permit respawning to other states.
In order to be playable in single player mode, a level must have a "Player 1 start" Thing. Player starts 2 through 4, typically located close by, enable cooperative play. Deathmatch start positions are usually located far from each other; one is chosen at random each time a player spawns.