Steven

Steven was an agent of the Knock Knock monster, who has been stalking Finn in his brain for a few months. He claimed that Finn was being used as an anchor of some kind, and that he needed to either co-operate with them - the child murdering demon monsters - or be killed. Steven seemed very offended when Finn chose the latter option.
He and Elthe struck up a great relationship, which had absolutely nothing to do with Steven being charmed. He revealed that he communicates by... I don't know ripping his skin off and shaping it around different ways, and has absolutely no way to communicate even simple ideas like where he's from. Considering his plan coming into this, he clearly isn't the sharpest.
We fight and predictable, Steven dies. He claims that his siblings will probably be along at some point to take us out, so we have that to look forward to at least.