Professor Tomoe is Hotaru Tomoe's father. He appears in Episode 90 of the anime and Act 25 of the manga. He is also second in command of the Death Busters.
In the manga Tomoe is just plain evil. After a lab fire killed his family and wounded his daughter Hotaru he joined the Death Busters of his own free will. Later in the manga he turns into a Germatoid so he could fight Sailor Moon, but he ended up being killed by her instead and unlike the anime, he does not come back.
In the anime he is a more sympathetic character. He displays bizarre, but hilariously overdramatic mannerisms, which are often a source of unexpected humor. Its never reviled if these are simply being part Germatoid or repressed aspects of his personality. His normal personality does show through at times, showing him to be a kind and loving father. At the end of Sailor Moon S, he is shown wheelchair-bound in a hospital with no memory of any of his Death Buster activities. He has taken it upon himself to re-raise the now infant Hotaru.
Professor Tomoe makes one more appearance in the anime, namely in episode 167 (the first episode of the Sailor Stars season), when Sailor Pluto arrives to take baby Hotaru from him. In this episode, he is seen without his glasses, and no longer relies on a wheelchair, indicating that his condition is improving. He is never seen or heard of after that.