Practical Game Design

The function of characters

Each character in a game should have a specific function, whether they are an ally or an enemy. You don't want to populate your world with characters that don't have a purpose and are just there to fill a blank space.

The same is true for enemies. Every time an enemy is introduced in a game, it has to serve a purpose in the overall game experience. Enemies are not just abstract entities on the screen trying to stop or kill the player somehow. Why are they trying to stop the player? Who are they? What are their motivations?

It is not important that all this information is actually conveyed to the player. It might well be that she would never know why those alien ships in Space Invaders were trying to invade our planet. ...