I like how he is looks like an evil villain; with his many arms, bald head, long claw-like nails, big moustache and small dark glasses. However, he could not be further from this. This is very clever character design, because not only does Sen feel threatened, but as an audience we are too. It is all a big relief when he turns out to be a great ally.
I also like his soot helpers. They are small and simply designed, they only have a fuzzy body, two eyes, arms and legs. Yet they still manage to get across a lot emotion and sense of character.