カチューシャ子 Kachūsha-ko
A recurring background character in Toaru Majutsu no Index. She acts as an easter egg in the series and generally appears in illustrations where Kamijou Touma's class from A Certain High School is involved.

She is referred to by Haimura as the background girl (モブ子 Mobu-ko?), "mob" referring to a minor character in a video game.
mob (toaru majutsu no index)