He is voiced by Chuck Huber in the English Version.
Being mad in science (hence the titular mad scientist he based on) he sees everything and one including himself as a possible experiment. This mental illness was suffered since his childhood: He often wanted to dissect the people around him only to be stopped by Spirit. This personality trait made all those around him afraid of him. This strange hobby of wanting to dissect anything he thinks is interesting was still present, even when Stein became an adult; once, as part of a lesson, he tried to have his class dissect an endangered animal much to Maka's concern, claiming that it would be bad if he did not find out anything from it before it became extinct. The stitching across his face suggests that he had worked on himself at one point.
He also exhibits a sadistic and intimidating personality. During his battle against Maka and Black☆Star, he grabbed Maka by one of her pigtails, lifted her shirt up and drew marks on her belly, to signify where he would put the knife through, and ponders if he should turn her skin into sandpaper. Also, when Black☆Star tried to attack him, he countered by knocking him out in a seemingly painful and bloody manner that her opponents believed that he killed Black☆Star, only for the latter turned out merely K.O.ed and yelling that he's still alive. His smiles are normally filled with sadistic intentions.
Despite this and his sadistic nature, he actually cares for and feels the need to protect those he considers his friends. He also exhibits a rather paternal demeanour towards his students and expresses deep disgust and moral outrage towards Medusa for stating that she would abandon her own child, Crona, when he/she would become useless to her. It is never stated when this other personality arose. Although Naigus stated that he always alone and had no love in him, Stein ironically capable in love as he had a romantic relationship with Marie and even had a child together with her. Although, given that his madness over science was suffered at his young age, feelings such as love in him may not have developed at a normal age, unlike normal people.
- His English Voice Actor, Chuck Huber is best known for voicing Hiei from Yu Yu Hakusho, Kururu from Sgt. Frog, Sleepy Ash from Servamp and Shou Tucker from Fullmetal Alchemist series.