Stein!

Stein is a tall man (approx. 193 cm), thin and pale-complexioned, with an attractive but unsettling face.
Hair: Short, silver-gray, and very messy.
Eyes: Green, always behind large round glasses.
Most iconic feature: A large metal screw driven into the left side of his head. He constantly turns it (usually clockwise) to concentrate or control his madness.
Body: Covered in black sutures and surgical scars all over the face and body (like Frankenstein's monster). A large suture stands out, running across the face from the forehead to the cheek.
Clothing: White lab coat (often with visible patches or sutures) over a gray patterned turtleneck sweater, white shirt, and black trousers. He almost always carries a cigarette.

Stein is the perfect archetype of the mad scientist with a touch of sadism and dark humor.
He loves to experiment and dissect anything he finds interesting (including himself and his friends… he even dissected Spirit while he was sleeping!). He sees the entire world as one big experiment or specimen.
He has a disturbing laugh (“hehehe” or “hahaha”) and likes to intimidate people as a joke.
He is eccentric, clumsy and witty: he moves around in his rolling chair, bumps into everything and has very funny moments.
Despite his apparent madness, he is extremely intelligent, analytical, and observant.
He constantly struggles against his own inner madness, but he controls it thanks to the screw and his willpower.
He is loyal to Lord Death and the DWMA. As a teacher, he is caring in his own way: he motivates his students (Maka, Soul, Black☆Star, etc.) with… unconventional methods, but he genuinely cares about them.
He has a hedonistic side and enjoys the pleasure of research and combat.