Professor Tomoe—usually just called "Professor" or "Doctor" Tomoe in the English dub—first shows up in Episode 90 of the anime and Act 25 of the manga. The two versions handle him pretty differently though.
In the manga, he's genuinely villainous. He willingly joins the Death Busters after a lab fire kills his wife Keiko and leaves his daughter Hotaru severely injured. Later he transforms into Germatoid to take on Sailor Moon directly, but gets killed by her Rainbow Moon Heartache attack. That's the end of him—no coming back.
The anime takes a softer approach. Tomoe becomes way more sympathetic, though he comes across as hilariously unhinged with these bizarre, over-the-top mannerisms that are honestly played for laughs. It's never totally clear if that's Germatoid's personality bleeding through or just repressed parts of Tomoe himself, since we don't know much about him before the possession. But his genuine self does peek through—he's actually a caring, devoted father underneath everything.
By the end of the S season, he's in a hospital wheelchair with zero memory of his Death Buster days. He's committed to raising Hotaru again, now that she's back to being a baby.
He makes one last appearance in Episode 167 (the Sailor Stars premiere) when Sailor Pluto comes to collect young Hotaru. At that point he's ditched the glasses and the wheelchair, suggesting he's making a real recovery. After that, he's gone for good.
Content compiled by AnimeList.moe from publicly available sources.
