This concept goes at least as far back as the seminal Gunstar Heroes. While most of the game is spent in traditional co-op, in one stage, the characters ride in a spaceship to attack an enemy space station. One player controls the spaceship, and is responsible for both player's survival, and the other plays a tiny, invincible gun turret, with more limited options and tasked mainly with protecting the ship player.
A more recent example of this idea can be found in the Mario Galaxy games. While primarily a single player game, a second user can jump in and control a reticle. As the reticle, player two can collect and fire star bits in the game world, aiding the first player.
Recently in DeathSpank, an action-RPG that sets player one as DeathSpank, a stereotypical fantasy hero, a second player can take control of Sparkles the Wizard. Sparkles is a supporting character that does not have an inventory or items, and can only use the same four abilities all throughout the game. He also lacks a health bar, serving only to protect DeathSpank. Sparkles does not even have an EXP meter, instead automatically improving in power as DeathSpank levels up. The hop-in-hop-out and casual nature of this character provides a simple, yet useful sidekick to the primary character.