Metal Slug

Secret Prisoner
In the first mission, after you blow up the wagon with bad guys coming out, you'll pass a building with one or two clearly visible prisoners (depending on how many players are playing). When you get between that building and the next, look up and you'll see a pair of legs hanging down. Shoot it and a prisoner will fall down. Save him, and he follows you around wherever you go until you lose one life. He is invincible and shoots Street Fighter-like fireballs that home into the enemy. He also gives you lots of bonus points if you finish the mission with him still following you.