@nobaka
Robara is a kitsune. I doubt Inari would allow a non-kitsune work for her. Ume doesnt have any physical kitsune traits as well(but has this weird illusion power thats different from the 'usual' illusions), some prefer to keep their human look while others, like Kon and Kaede, go wild easily.
I think the reason he looks like a human is because he and Hii-chan seemed to grow up in a human-like kitsune(?) society.
Umetarou is also well-accustomed to the human society, so he's the most 'human'-like. On the other hand, both Kon and Kaede grew up in the wild.
^from the extras
Roba's powers: He's a genius. Unlike Inugami's transformation, Roba's has 'durability' - meaning, that the transformed thing itself has no problem keeping the form after, well, transformation (For example, Inugami and others can't keep the transformed things in such state for a long period of time). Inugami uses his 'will' to transform something into something, so his transformations break easily when he's away. Like the transformed Life Calculus from the earlier chapters. Roba's glue is not connected to his will.
^also from the extras
I think its better to say there's two types of transformation: the one that every kemono can do (they transform into their human/kemono forms) and the other is the speciality of tanuki (and Robara) (transforming something into something)
And yes, Megijima says akuma - devil (as in, western thing) to differentiate from oni ('demon').