In single-player, you take control of Luke with Ruby as the second member of your squad, while in co-op (online or off -- jump in/out at any time) the second player controls Ruby directly. You have light squad control features in the single-player mode along the lines of a Gears of War, but the game does a few things you might not expect if you think of it as a "squad" game.
For one, the two characters will show their affection for each other as you play. We witnessed one scene where Luke had Ruby provide cover fire for him as he ran around to the side to flank an enemy. Once the area was clear, Ruby walked up to Luke and gave him a kiss as a reward for a job well done. This has gameplay ramifications as well -- if you "die" and Ruby stays alive, she will run over to you on the ground and give you a "kiss of life," which brings you back to your feet.
Sounds like a little more elegant way to revive someone instead of the Kane and Lynch method of pounding adrenaline into your heart. It appears though, that without a publisher, the game is looking at a 2009 release...a late 2009 release. We'll be keeping a close eye on this one for sure.