We already knew that "Super Mario Galaxy 2" would have co-op. After all, the first game did, what with the whole "assist" element using another remote. It wasn't exactly involved or, for that matter, helpful, but it did make the game feel a bit more multiplayer.
Seems that Nintendo liked the idea so much that they'll be improving on the co-op in some way. The above screenshot was taken from the official Nintendo site for Japan and seems to show that a second player will have direct control of one of Mario's star-shaped friends. Details don't go much beyond that above screenshot, but it does seem to imply a more involved co-op experience, since the second player actually has a character to control.
Given that "Super Mario Galaxy 2" is nearing release, I'm hoping that all will be made clear soon. In the meantime, I'm just hoping that Birdo makes a cameo. Who doesn't love Birdo?