Osamu (unnamed in the dub) is Jaden Yuki's childhood friend whom he played Duel Monsters with while his parents were busy working (in the Japanese version, Jaden calls him "onii-san" which translates into "big brother", which in this case is used as a term of affection). One day, after Jaden lost to Osamu, Jaden grew upset over the fact he couldn't summon his favorite card, "Yubel" during the Duel. Osamu thought he was overreacting and tried to comfort Jaden, but ended up evoking the wrath of Yubel by inadvertently insulting her (in the dub, Yubel attacked Osamu for saying that Jaden was crazy because he claimed the Yubel card talks to him). He went into a coma and was transported to a hospital. When Jaden went to see him at the hospital, bringing a small bouquet of flowers as a gift, Yubel inflicted further pain on Osamu.
During Nightshroud's invasion, among the silhouetted images of Duelists that Nightshroud shows to Jaden is one that resembles Osamu.