Mizuguchi Yuto, known professionally as U, is a Japanese singer, dancer and member of ONF (온앤오프; short for ON N' OFF). Before debuting, he was a trainee at JYP Entertainment for one year, and after switching companies, he was introduced as part of WM Boys with their first appearance as B1A4's backup dancers at the 2016 Dream Concert in Seoul. He gave a performance, alongside his groupmates, at IDOLCON just a few months before debuting. He made his official debut in August 2017 as the group's OFF Team member, main dancer, sub-vocalist and current maknae with their 1st EP "ON/OFF". In 2018, the group signed a contract with Victor Entertainment (ビクターエンタテインメント) and made their Japanese debut and held a concert in Tokyo. In 2020, all the members entered MNet's "Road to Kingdom" and finished in 2nd place, and he had his acting debut with a role in the web drama "Can I Step In?". In 2021, all members (except for him due to his Japanese citizenship) decided to enlist for their mandatory military service around the same time. The following year, he entered "Be Mbitious” in order to be selected as part of Rain's crew that was going to compete in "Street Man Fighter", but he was eliminated in the preliminary auditions. He returned to Japan and spent time with his family while waiting for the others to be discharged from the military. After their release in June 2023, the group became active again. They are represented by WM Entertainment (WM엔터테인먼트).