Abaddon placed his claws on Serah's wounded shoulder, allowing some of his symbiote to slither off of him and onto her. It wasn't going to bond with her or anything, merely remove the arrow sticking out of her and slowly heal the wound. Why help her? Simply because she was as lost and disoriented in this new world as he was, although Abaddon was starting to map out the area little by little. Plus, Serah didn't seem like a villain, at least not one like Apollyon. 

"Better?"