Keisuke is generally a kind man who cares for his family and he was saddened by Mahiru Sakai's death, but he dealt with it to the point he managed to raise Wakana Sakai 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.
Keisuke plays a small role early in the series as he is shown in flashbacks with Mahiru Sakai and Wakana Sakai (Keisuke's daughter), during the time that Wakana is repeatedly remembering her past and her mother.
Mahiru was Keisuke's wife who was shown to love her dearly as he did his best to provide for her and Wakana Sakai before her death. According to Keisuke, Mahiru was the one to propose marriage using the piano that Wakana has in her room.
Wakana is Keisuke's daughter whom he deeply cares for and 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 Sakai's illness so they could spend more time together, but Mahiru disagreed. When Wakana fell ill with a cold, he hurried home from a job to check on her.
- The name Keisuke means "open, begin" (啓) (kei) and "forerunner, herald" (介) (suke).
- Keisuke's surname Sakai means "liquor, sake, alcohol, booze" (酒) (saka) and "well, mine shaft, pit" (井) (i).
- According to Keisuke himself, he was a bit of a ladies man before he met Mahiru Sakai.