Professor Souichi Tomoe is not a reassuring figure. He has a fairly normal frame, tall and slender with broad shoulders (usually dressed in a lab coat), and a shock of white-blond hair. However, one of his eyes has been replaced by a faceted jewel; the overall impression of his face is strange and chillingly alien. According to Usagi, his hands are inhumanly cold.
He is ruthless, unprincipled, and intelligent; his goal in science is to meld life with nonliving objects to create the "superhuman." Since he would not stop performing experiments on animals, he was cast out of the academic community. To make a living, he put his experimental research up for the highest bidder; it was bought by the Mugen district. Two years later, while the Mugen buildings were being constructed, a fire broke out. His wife Keiko died, and his eight-year-old daughter Hotaru was critically injured.
Despite Tomoe's faults, he was a caring and loving father and husband, and part of him died that night. He comforted Hotaru as best he could, and rebuilt her body with wires and machinery, making her a cyborg.
While Tomoe was rebuilding Hotaru's body in his labs, lightning struck down from the sky and hit Tomoe's assistant Kaoli. Kaoli screamed as she fell, then got up, her eyes shining strangely. Kaoli was now Magus Kaolinite, a creature of the Death Busters. She showed Tomoe that he could implant daimon eggs inside people. Tomoe happily implanted a special one inside little Hotaru, and started building daimons himself. Tomoe believed himself chosen by God to perform these special tasks for Pharoah 90.
When the Mugen buildings were complete, he moved his labs into the direct center of the Mugen district, and founded Mugen Gakuen. Mugen Gakuen's reputation as an elite school drew talented students by the hundreds, providing more innocent souls for Pharoah 90, and more useful bodies in which Tomoe could plant daimon eggs. While Kaolinite and the Witches 5 fought off the senshi, Tomoe was able to continue his work undisturbed.
Finally, the form of Mistress 9 took over Hotaru's body, and began opening the doorway for Pharoah 90. Supersailormoon and the outer senshi burst into Tomoe's underground laboratories, and he threw daimons at them. When the daimons were destroyed, he turned himself into a huge monster with a star on its forehead. He called himself Gelmatoid, the superhuman, and the triumph of all his research.
Soft-hearted Usagi was unwilling to attack Gelmatoid, because he had once been Hotaru's father. However, Sailoruranus had no such scruples. She used her Space Sword on Gelmatoid, but he was unaffected and knocked her easily aside. Then Usagi knew that she had no choice, and with her cry of "Rainbow Moon Heartache," Professor Tomoe was destroyed.
As he died, Hotaru's spirit looked on. Papa... she whispered. The gentle papa when mama died... The papa who rescued me... He wasn't my papa anymore...
Professor Tomoe was a raving lunatic, and a literally mad scientist - there is no doubt about it. However, I personally don't think that he deserved to die. He was once a loving father and husband, and could have been again. This is one of the points in which I actually like the anime better than the manga. Sure, it's cool that the outer senshi get to raise Hotaru - but Tomoe deserved a second chance, and he got that in the anime.