Jules Yun is the son and heir of Vernon Yun's estate. He has a strained relationship with his father after his mother died and he is not very comfortable around Isaac Yun. He is a polite, emotional young man who is not a very strong fighter.
He was the first to realise, albeit by mistake, about Isaac being a robot. Along with his group of friends he was contained during the Atebus Masquerade Murder on suspicion of murder.
He is best friends with Garret Fox and has been since they were both young. Garret's affair with Adriette Lockwell almost broke up their friendship as Jules has always had feeling for the woman. Now they run Yun Enterprises together, creating Steamwork pets for users.