This was one of my favorite features of Kirby Super Star for the SNES - you could summon a "helper" with special powers. If nobody was using the second controller, the game would control your helper, but the moment somebody pressed a button on the second controller, the helper was controlled by the second player. There are some places, especially in The Great Cave Adventure, where you NEED the help of the second player to collect all of the items (but not to beat the game, which can be done without collecting all of the items.
The helper couldn't "run out" of lives, though they could die, though it was a single button press by the first player to remake them. Items that regenerated health could be shared with the helper to regenerate their health an equal amount.