Or Osanai has been influencing him too

I’ve also thought about this possibility, but it weirds me out that Urino became so bold with Osanai right after Hiya told him to be assertive with her. Before their conversation Urino was intuitively cautious enough to not overstep her boundaries.

Sometimes I think Hiya’s too suspicious to be such an obvious suspect, which is kind of suspicious in itself. Maybe not all of the arson cases were done by the same person. The first fires could have been a coincidence used as a cover-up to burn the van, and it would be easier to find some kind of pattern between just a few cases and deem it serial arson, like Urino did. Or someone tested the waters with those fires, Urino took the bait, and they decided it was safe to burn the van so no one thinks it was the initial target. Anyway, I hope Kobato solves the case soon enough, so my brain stops with all these theories.