Ao no Exorcist: Yosuga-hen Episode 1

Unlike his twin brother, Yukio, Rin Okumura never wished to know about their parents and the circumstances surrounding his own birth. Yet the master of time, Mephisto Pheles, gives Rin a key that transports him 40 years into the past when his late mother, Yuri Frederick Egin, was merely a child. With Mephisto's guidance, Rin leaps through the years of Yuri's childhood and her adolescence as an exorcist-in-training.