Y4-5U (Or Yasu (Pronounced Yah-soo)) was accepted into Hope's Peak as the Ultimate Reploid for being able to communicate with humans so normally people mistook him as a human. He's a nice mechanical person to be around, often helping Saburou with chores, such as cooking or cleaning. He tries to hide an evil personality but he only fails to conceal it when he comes in contact with a dead body. He has a medium-pitched voice.