Oh shit. Already?>>105018
Keiki is a goddess of "crafts", which should be interpreted broadly as technology and creativity. (Clay and pottery was hi-tech at some point.)
She granted creativity and technology to the primates in the Beast Realm (one of the Buddhist hells, where bad people are sent before being reincarnated as animals), and with their newfound gifts, the primates were able to drive away the other beasts and build an advanced civilization.
She's the final boss because beasts spirits escape from the Beast Realm and into Gensokyo, where they trick the protagonist into helping them fight Keiki, at first by deception and eventually by possessing their bodies. Civilization fails, and the wild beasts reclaim their realm.
Please remember Keiki after her thread is gone!