Nemuri Kayama (香か山やま睡ねむり Kayama Nemuri?), also known as the R-Rated Hero: Midnight (18禁ヒーロー ミッドナイト Jū-hachi-kin Hīrō Middonaito?),[1] was a Pro Hero and faculty member at U.A. High School, who taught Modern Hero Art History.

Nemuri was a tall, curvaceous woman with sky blue eyes that tilted downward in the center and were framed by a set of rather long eyelashes. She had long, thick, spiky, dark purple hair, which was made up of layers of varying lengths, the longest ones reaching down below her waist. Her chin-length bangs were split into three sections: two swept to the sides and one over her face, going diagonally down to the left between her eyes. She also had a small beauty mark below her left eye, as well as red-painted nails.

Her hero costume was remarkably similar to one of a traditional dominatrix. She wore a breastless, black leather leotard—similar in style to a corset—over a thin, skin-tight, white bodysuit (flesh-colored in the manga), which accentuated her voluptuous figure. The center strip of her leotard was embellished with red gemstone-like accessories in a vertical pattern, extending from her collar to her midriff. She also wore translucent, black, thigh-high garter stockings, which were attached to a burgundy utility belt decorated with gold studs around her hips, a matching pentagonal buckle in its center, and black knee-boots. She had a small red mask outlining her eyes and a handcuff on each wrist. She was often seen carrying a flogger-style whip. The white bodysuit was made of thin material that is easily tearable, so she could more easily use her Quirk. Nemuri had stated in an interview that she was trying varied materials to find out what works while adhering to costume regulations. In Vigilantes (and briefly in Chapter 24), she is also shown wearing stud earrings. The Vigilantes anime adaptation, and some colored artwork of its source material also depicts her with reddish-pink lipstick while in costume.

During her years at U.A., Nemuri's hair was much shorter, having reached down to her chin. Her original costume was also much more revealing. Her breasts were covered by black protective plates, and she wore a dog collar, along with only undergarments, which were covered with a utility belt that held her whip. She was otherwise completely bare aside from an unbuttoned, high-collared trench coat. She also had large, angular shades, different from the mask she wears now.