- Voiced by: Mitsuru Miyamoto (drama CD), Yuuya Uchida (Japanese), Chuck Huber (English)
- A mad scientist who works as a teacher at DWMA. His name is a reference to Mary Shelley's novel/character Frankenstein. He views everything in the world as a specimen to be experimented on, including himself, leading him to drive a large screw through the side of his head and interweave his own skin, clothes, and even his laboratory with stitches. He normally serves as a kindly father figure to his students, though he suffers from chronic bouts of madness that compel him to try and dissect anything in sight, wearing a twisted and sadistic smirk during these intervals. Other than being a scientist, Stein is considered to be the most powerful meister to ever graduate from DWMA. His greatest strength is using his Soul Perception to read his opponents' souls and use the data he acquires against them. He works well with many weapon partners, including his childhood partner and Shinigami's current weapon Spirit (who he also experimented on in his sleep) and his old flame Marie Mjolnir. Even without a weapon, Stein is a master martial artist and can unleash potent attacks with his soul wavelength alone, making him a deadly opponent in any situation.
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