[@Gelatinous Cube] No need to apologize compadre. To my knowledge you can essentially co-op the entire game except for the level leading up to the first boss. The co-op though works really weirdly (but is a cool twist) as you have to either use this expensive item called an ember or defeat a boss in order to be able to summon people. You also loose your co-op partner if they die, you die or you beat a boss.