He lived in Hasunobe during the time of the First Large-Scale Invasion.[citation needed] Rinji Amatori was investigating Neighbors during the days before he went missing, resulting in Osamu owing him a favor of looking after Chika for him if he wasn't able to anymore. Due to this, Osamu attempted to join Border, but he was not accepted. He tried to sneak in, but a Trion Warrior attacked him. Jin saved Osamu from it.