Otoya's Aura should be an owl.

Otoya's characteristics are his speed/agility, silent steps and ninja thematic.

Owls have totally silent flight and they fit for a ninja animal. Also that would make a nice parallel with Karasu.

I also think a few other characters would benefit from an beast type aura, like Yukimiya who recently got predator eyes (a jaguar, maybe a black one, would be fine, although it would probably be too similar to Chigiri).