Umi Monogatari: Kanon Miyamori

Kanon is a sky dweller who, after doubting her relationship with her then would-be boyfriend Kojima, throws her ring -- a gift from him -- into the sea. This eventually leads to her meeting Marin, which leads to Urin releasing Sedna and Kanon becoming the priestess of the sky. Together, they fend off the minions of Sedna to the best of their abilities.