Aki Hayakawa was a Public Safety Devil Hunter working under Makima's special squad. He had a contract with the Fox Devil in the past. Also, the boy has an agreement with the Curse Devil that grants him a nail-like sword. He can summon the Curse Devil to seize the enemy and inflict massive damage upon them, potentially killing them during the process. That's what he is doing in our fanart Chainsaw Man Aki Hayakawa Summons a Devil Sticker.