Practical Game Design

Chaser/Aggressor

This is the most common type of enemy. As soon as the player is in sight, its only objective activates: to kill him. It will chase and attack the player by any means possible until one or the other is dead. You can find this type of enemy in very complex games, such as MMOs (where they decide who to attack depending on how aggressive they are and how much of threat the players nearby are), or it can be more static, like the alien ships in Space Invaders.