To be fair, he pretty much completed his end of the bargain by getting the children healed. The rest was just extra service. And the contract seal on Yona seems to be the whole doing anything in return part. I'd imagine he could just return to his lake/physical manifestation of the remainder of his being, but that would also be just him up and leaving rather than placing the children somewhere safe/physically relocating them closer to his lake.This water spirit is certainly a pushover... He's been contracted to someone, and yet he's already responding to emergency requests from other people and trying to earn tons of money. What's the meaning of this contract concept if the spirit can just run off?