The Purple Orca captain's design reminds me of Mermaid Man.
So far, William is hinted so strongly that he could be the traitor, but I think that's too easy. I'd love to have some other people being the traitor, and William actually cooperating with Yami to track the real villain.
Given Aster's reaction by looking away from Yami, he already knew who the traitor is. I hope it is not Yami. Or Julius could pull a trick to accuse other captain besides the traitor, so that he/she feels safe. That's when Julius strike him/her for good.
but for now, we can only enjoy the cliffhanger.