No one; from what I have read, it evolved with the people of Japan. Shinto(ism) also has no official formalized system of doctrine or sacred texts. There is debate on the time period when Shinto originated,

there arent any they dont have any written texts

I think they had scripture early on during the time the emperor was both religious and political leader than a shogun. but the shoguns just incorporated Buddhism for happy occasion like weddings, births, summer holyday, and Shinto was just solemn to shrines for other things. the general rules about Shinto is belief in a certain pantheon and that the emperors' line in the direct bloodline of the goddess Amaterasu. Since the emperor would be the most accessable "god" on hand, he had ultimate control. and some particular holysites, shrines, and like great epic battles.

Yoshida Shinto