Hagi was a young street performer until one day, he was assigned to be the guardian of Saya Otanashi
in 1873, when she had no knowledge of what she is or the experiment that she was in since her birth. He provided her daily "medicine" (a glass of blood) and helped Saya learn to play a Cello throughout his duties. Years later into his adulthood, he learned of Saya's healing abilities after an accident during cello practice, in which he was kicked out after witnessing it. Days later, the two confirmed their relationship as potential lovers after Hagi barged in Saya's room after getting her overdue "meds". Unfortunately, he was critically injured by Diva
after she was released from her prison thanks to Saya, whom later revived him as a Chevalier after her sister's rampage. Hagi decided to go alongside Saya to kill Diva. Since then, Hagi had been involved into The Red Shield's motives centuries later while resuming his duties as a guardian to Saya after she recovered from her years-long amnesia.