Professor Souichi Tomoe was the father of Hotaru Tomoe, owner of Mugen Gakuen, and a famous scientist. After an accident at his laboratory, he became a key part of the Death Busters.
In the manga, Tomoe was a scientist until, as Kenji Tsukino recalled, he was thrown out of the scientific community for inappropriate experiments. After selling some of these findings, he purchased the area where he built his laboratories and Mugen Gakuen. He continued with his research but a lightning strike caused a fire there, which killed his wife Keiko and left his daughter Hotaru badly injured.
Souichi saved his daughter using what appeared to be a mix of cybernetic and genetic technology. He also implanted a Daimon Egg in her at this time, the Eggs having arrived with Pharaoh 90 during the lightning strike. They infected his assistant, Kaori, as well as himself.
After this he focused on the creation of his "Super Beings," using a variety of scientific techniques including genetic, pharmacological, and cybernetic processes. Human bodies were used as vessels, but he also made several Daimons from the Eggs alone: the Witches 5. He experimented on himself as well, creating a fusion of human and alien.
When Super Sailor Moon arrived at his laboratory with the Outer Senshi, Daimons attacked and Tomoe changed into a creature called Germatoid. Sailor Moon killed him with Rainbow Moon Heart Ache, and Hotaru was briefly seen saying goodbye to her father, whom she realized had disappeared a long time ago, after her mother's death.
Professor Tomoe first appeared in episode 90 of the anime. As in the manga, his wife was killed and Hotaru severely injured in an explosion at his laboratory, and he allowed himself to be possessed by Germatoid (who was a separate entity in this continuity), and Hotaru to be possessed by Mistress 9.
After the battle with Pharaoh 90 and Hotaru's rebirth, Professor Tomoe was left with no memory of his past. Baby Hotaru was left in his care until episode 167, when Setsuna came to retrieve her, and after that he was not seen again in the series.
Despite being possessed, and acting as a major antagonist for much of Sailor Moon S, he appeared to be kind and considerate of his underlings (despite occasional frustration with them), and at times even encouraged them or engaged them in friendly small talk, in contrast to other villains in the series.
- Although he was a villain in Sailor Moon S, Professor Tomoe later appeared as a civilian in Sailor Stars.