Youji is a young boy with brown hair and brown eyes. He and his sister look very much alike. He wears a red sweater over a yellow shirt, grey pants, and brown shoes.
Youji is the more shy twin of the two. He is a very sweet boy, and takes to calling both Sumire and Lucia "onee-chan" after they save him. Later, when Lucia isn't feeling well, he writes her a letter that Sumire delivers, wishing her well.
Youji makes his first appearance in Chapter 4, after his twin sister Tsukuyo comes to Venus Vanguard to ask the girls for help, telling them that her brother is plagued by a monster and won't come out of his room. After taking the case, it is later discovered that Tsukuyo was attacked by a virus and was becoming a virus herself. Fortunately for her brother, she cared so much about him that she sought help for him, even though that would lead to her own death.