Genkai

Genkai (幻海, Genkai ) is an elderly martial arts expert who lives alone in a very large temple. She is the former lover of Younger Toguro, who eventually becomes a demon after competing in the Dark Tournament. She trains Yusuke and the other after Yusuke wins the tournament to decide her successor. She joins their team in disguise during the Dark Tournament in order to help Yusuke. She eventually transfers most of her power into Yusuke, which leads to her death while fighting Toguro. Yusuke wishes her back to life as his reward for winning the tournament. She helps Yusuke battle Sensui and prepare to leave to the demon world afterward. She eventually dies and leaves her temple to Yusuke and his friends.

She is the creator of the Spirit Wave (修の拳}, Shuu no Ken, lit. Fist of Mastery), a powerful technique capable of eradicating evil energy. By manipulating her energy, Genkai can temporarily revert to the state of a twenty year old woman, which is when she was at the peak of her power. She is voiced by Hisako Kyōda (Older) and Megumi Hayashibara (Younger) in the Japanese anime and Linda Young in the Funimation dub.

Source: Wikipedia