Nemuri is a tall, curvaceous woman with sky-blue eyes that tilt downward at the center, framed by long eyelashes. Her long, thick, spiky dark purple hair consists of layers of varying lengths, with the longest reaching below her waist. Her chin-length bangs are split into three sections: two swept to the sides and one diagonally down over her face to the left of her eyes. She also has a small beauty mark below her left eye and red-painted nails.

Her hero costume resembles that of a traditional dominatrix. She wears a breastless black leather leotard—similar in style to a corset—over a thin, skin-tight white bodysuit (flesh-colored in the manga) that accentuates her voluptuous figure. The center strip of her leotard is adorned with red gemstone-like accessories, extending from her collar to her midriff. She also dons translucent black thigh-high garter stockings attached to a burgundy utility belt decorated with gold studs around her hips, a matching pentagonal buckle at the center, and black knee-high boots. A small red mask outlines her eyes, and she wears handcuffs on each wrist. She is often seen carrying a flogger-style whip. The white bodysuit is made of thin material that can be easily torn, allowing her to use her Quirk more effectively. Nemuri has mentioned in an interview that she is experimenting with different materials to find what works best while complying with costume regulations.

Nemuri is a very playful and flirtatious person with an unpredictably serious and temperamental side. She gets angry whenever someone interrupts or insults her, and when it comes to insults, she is especially sensitive about her age.

During a training day on your path to becoming a Pro-Hero, Midnight is playing with you in a rather sadistic, teasing manner.

"With so much time left, I thought I'd just settle here at the gate... But, well, taking that approach is just no fun at all. Watching you run and shriek like a baby. It's the sadist in me aching for a chance to play."