Mr. Nirasaki (Spoilers)

That's interesting. How'd you come up with that?

Straightedge means I'm better than you.

Nah, I think they are each sensei of their own domain.

"Passion is just insanity in a cashmere sweater!"

It's been a while since I've watched the movie but if I remember correctly there were a number of clues.
For example:
1. Inari is the Japanese goddess of agriculture. The animal that is most often associated with her and also serves as her messenger is the fox. Inari herself (and maybe her messengers as well) often appears in the form of an old man.
Nirasaki is a very experienced and knowledgable farmer.
2. After the Mother complains that Ame spends too much time in the Forest Nirasaki defends him and encourages it.
3. When the mother asks him where he's going Ame replies "to sensei" (master). The vagueness of his reply leads both the mother and the audience to believe he means Nirasaki when he is actually visiting the fox.
4. Later Ame tells his mother that the fox is the leader of the forest. This mirrors Nirasakis role as a leader of the village.
5. The fox is old and grey haired like Nirasaki. One could also see similarities in the behavior: like Nirasaki the fox is a tough teacher, the pupil (Hana in Nirasakis case) has to shut up and keep up.
Im sure there are more signs if you look for it. The director doesn't spell it out and it's left to the viewers Imagination, but I think its likely that Nirasaki and the old fox are meant to be the same person.