Team Ico's aptly-titled debut Ico is a gorgeous adventure game in which players take control of the titular young boy who is trapped inside an abandoned castle.
Ico spends the game attempting to escape alongside a captive girl, Yorda, and due to Yorda's limited physical abilities, players must help her traverse the castle by providing boosts and manipulating the environment.
Shadow creatures will also attempt to drag Yorda away if you leave her unaccompanied for any amount of time, so you need to be constantly watching and assisting - or if you want to be a bit mean, babysitting - the poor girl.
Yet the ultimate expression of Ico's gameplay is only unfurled once the game has actually been completed, when the European and Japanese versions of the game will unlock a co-op mode, allowing a second player to take control of Yorda.
Granted, she's still subject to all of the physical limitations that her CPU counterpart is, but allowing another human to take control of her adds an undeniable visceral quality to gameplay, especially if playing with a loved one or perhaps even someone who isn't terribly experienced with video games.