This show is fairly peculiar even by director Shinbou Akiyuki's standards. The first episode reminded me of Shinbou's 2004 gothic horror OVA, Le Portrait de Petite Cossette, in which his art-school origins first (?) revealed themselves.
The main cast is composed of Shinbou favorites: Asumi Kana, Saitou Chiwa, Hanazawa Kana, and Nonaka Ai. Sasami's brother is played by veteran seiyuu Ootsuka Houchuu, who had roles in Zeta Gundam, Slam Dunk, and Naruto.
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Sasami is the successor priestess to the shrine of the moon god Tsukuyomi, but she also bears within herself the powers of his sister, the great sun goddess Amaterasu. Wanting nothing to do with all this, she has fled to the life of a hikikomori, spending all day in her room, in her pajamas, on the Internet. Her brother -- whose face we never see -- slavishly devotes himself to looking after her every need.
The other recurring characters are three sisters whose names refer to the three sacred treasures of the Japanese Emperors: the mirror, sword, and jewel (Kagami, Tsurugi, and Tama). The smallest sister, Tsurugi, is the eldest; and the biggest, Tama, is still a grade-schooler.
I was entranced by the show's ultra-Shinbou visual fantasies in episode one, but since then its wonderful mood, pacing, and characters have taken over for me. The show does not appear to be popular either in Japan or abroad, but personally I love it. Seems like Art to me.