"Berzerk is a multi-directional shooter video arcade game, released in 1980 by Stern Electronics of Chicago.
The player controls a green stick-figure, representing a "humanoid." Using a joystick (and a firing button to activate a laser-like weapon), the player navigates a simple maze filled with many robots, who fire lasers back at the player character. A player can be killed by being shot, by running into a robot or an exploding robot, coming into contact with the electrified walls of the maze itself, or by being touched by the player's nemesis, "Evil Otto."
The function of Evil Otto, represented by a bouncing smiley face, is to quicken the pace of the game. Otto is unusual with regard to games of the period, in that there is no way to kill him. Otto can go through walls with impunity, and is attracted to the player character. If robots remain in the maze Otto moves slowly, about half as fast as the humanoid, but he speeds up to match the humanoid's speed once all the robots are killed. Evil Otto moves exactly the same speed as the player going left and right but he can move faster than the player going up and down; thus, no matter how close Otto is, the player can escape as long as they can avoid moving straight up or down."
Source: Wikipedia, "Berzerk," available under the CC-BY-SA License.