Nine Seragaki

Nine met Aoba along the shores by the church Aoba was left in care to. Nine first tells Aoba about his interest in traveling and the ocean before mentioning that he used to be just like Aoba. He tells Aoba that there are no wrong options in the world, and that at any time he could destroy the world and build a new one. Nine then decides to 'build a new world' for Aoba, by giving him a name and adopting him. It is later revealed that Nine met Haruka in Midorijima, finding her alone on a beach, just like Aoba, and helping her create a new world with him as well. Nine is known for his constant traveling, saying that he must hear different voices. It is also revealed that Nine was previously a test subject for Toue who managed to escape, becoming another reason for his travels.

Aoba reasoned that if Naine had stayed with him in Midorijima to be his father, he would not longer be himself. For this same reason, Aoba accepted his parent's departure, despite the fact he felt sad after they left the island. He believes, however, that they are going to come back to him one day.

(https://dmmd.fandom.com/wiki/Nine_Seragaki)