Tsukoyo Nogami is the twin sister of Youji. She requested the help of Venus Vanguard to protect her brother from the 'monster'.
Tsukoyo is a very sweet little girl, and is the more outspoken twin. She has always looked out for her brother Youji, and she cares a lot about him. So when he doesn't want to see her all the sudden and hides in his room, she is very upset and seeks help so that the monster will leave her brother alone. At the conscious level she hadn't yet figured out that she was the monster he was hiding from.
Tsukoyo is a little girl with long brown hair and brown eyes.
Tsukoyo appears in Chapter 4 of the manga, when she comes to Venus Vanguard to ask for help protecting her twin from the 'monster' (a virus). It is later discovered that she was attacked by a virus when she and her brother were at the park. Youji ran away and left her there. When she returned to the house, he was the only one who could see her (she thought everyone else was ignoring her) but he wouldn't let her come near him, because he knew she wasn't human anymore. She finds a flyer for Venus Vanguard and went to seek their help. She is killed by Lucia once she mostly transforms into a virus and tries to attack Sumire.