Ao no Exorcist: Yosuga-hen

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.