Souichi Tomoe was the highly gifted and eccentric father of twelve-year-old Hotaru Tomoe. He was a very talented scientist in engineering and astrophysics. Upon being fully taken over by the Germatoid Diamon within him, he was killed by Super Sailor Moon's extremely powerful Rainbow Moon Heartache attack.
Several years ago, Souichi Tomoe was a very knowledgeable and brilliant scientist in bio-engineering and astrophysics. He believed that alien life-forms existed and that merging them with human beings would create an entirely new kind of being: a super life-form. When a fire broke out, he had managed to save his six-year-old daughter and then experimented on her by infusing her physical body with machinery to ensure her well-being. He was approached by Pharaoh 90 himself who had him declare his undying allegiance and devotion to the cosmic entity in return for great power and knowledge far beyond humanity's. Although hesitant, he willingly devoured the Diamon egg of Germatoid and thus become completely possessed by his evil.
Season Three: Death Busters
He had scolded his daughter for being rude to his trusted assistant, Kaorinite, but soon let it slide as he had spoiled her since her mother had died. At his house, he was introduced to Chibiusa who he greeted warmly before disappearing to his laboratories to continue his experiments.
His true intentions are revealed when he first encountered "the alien" during one of his experiments. He then pledged his loyalty and devotion to this universal entity and was transformed into an entirely different man. He then used his scientific genius to effectively create "perfect bodies" which were fusion of human and machine by genetic manipulation; and such had artificially created the other four members of Witches 5 in his lab.
Believing himself a god, he turned his hordes of Diamon on Super Sailor Moon, Super Sailor Uranus, Super Sailor Neptune and Super Sailor Pluto, who fought back. As he was Hotaru's father, Super Sailor Moon was quite reluctant to either attack or kill him. It was then that he become one with Germatoid, and became tens of times much stronger and attacked ferociously. According to Super Sailor Uranus, he was incapable of returning to human form. Sailor Moon killed him with Rainbow Moon Heartache, and Hotaru was briefly seen saying goodbye to her father, whom she realized had disappeared a long time ago, after her mother's death.
After devouring a unique Diamon egg called "Germatoid," he gained the ability to create highly intelligent life-forms with ease by combining organic life and machinery. He was successful in making hundreds of Diamon eggs for his beloved master.
As a complete Diamon, he possessed superhuman strength and superhuman durability, capable of knocking away Super Sailor Uranus' Space Sword from her hand, and casue cracks to appear all around his lab.
- While his new form is considered Germatoid, the name itself is not mentioned in Sailor Moon Crystal.
- His original anime counterpart was not killed. He lived to care for his infant daughter, who was reborn along with the spirit of Sailor Saturn, until she was taken by Setsuna Meiou in the Season 5 premiere.