Keisuke Sakai is Wakana Sakai's father and a side character to the series.
Keisuke is generally a kind man who cares for his family. He was saddened by his wife's death but he dealt with it to the point he managed to raise Wakana to the best of his ability. He is dedicated to his work and providing an income for his daughter, often taking on side jobs, such as working on a farm, in order to earn a decent wage. According to him, before he met Wakana's mother he was a bit of a ladies man.
Keisuke plays a small role early in the series. He is shown in flashbacks with his wife, Mahiru, and daughter, Wakana, during the time that Wakana is repeatedly remembering her past and her mother.
Wakana is Keisuke's daughter whom he deeply cares for. He works hard to earn an income for their small family. During the time before his wife's death he felt it would be better to tell Wakana about Mahiru's illness so they could spend more time together, however Mahiru disagreed. When Wakana fell ill with a cold he hurried home from a job to check on her.
Mahiru was Keisuke's wife before her death a few years earlier. Keisuke was shown to love his wife dearly and did his best to provide for her and their daughter. According to Keisuke, Mahiru was the one to propose marriage using the piano that Wakana has in her room.