The symbiote unlocks the arousal bar and when it hits 100 you transform into him. When you're in that state you can use one extra special attack with move linking * which means it's a wild card. There's also an extra fight move that builds arousal but it's not very useful. Turning into the symbiote gives you some extra stats and hit points I think, but it seems almost negligible after all of the other abilities that have been unlocked. When the limit break system went in, it wasn't enabled until you got the symbiote but now it's just immediately available. As a result, you can pretty much ignore the symbiote.