When playing with a friend, your partner takes control of Igiculus, a ball of light that can interact with the environment. Not only can he illuminate the way, he can open up new paths and stall enemies. In combat, Igniculus can also interrupt enemy attacks. However, these abilities can't be spammed--player two must scour the environment for orbs that refill a power bar that enable these powers.
Co-op isn't the most involving feature of Child of Light. However, it's a way to introduce a partner to what would otherwise be a solitary RPG experience. Check out this new trailer to see co-op in action: