Souichi Tomoe is generally referred to simply as "Professor Tomoe", and the English dub refers to him as "Doctor" Tomoe. He first appears in Episode 90 of the anime and Act 25 of the manga.

In the manga Tomoe is outright malevolent. He joins the Death Busters of his own free will after a lab fire that killed his older wife, Keiko and seriously wounded his young daughter Hotaru Tomoe. He later turns himself into Germatoid to fight Sailor Moon but is killed after being hit by Sailor Moon's Rainbow Moon Heartache. He dies in the manga and does not come back.

In the anime he is a more sympathetic character. He displays bizarre, but hilariously over dramatic mannerisms, which are often a source of unexpected humor. It is never specified whether these are mannerisms of Germatoid or repressed aspects of his personality, since little is known of him before or after the possession. His normal personality does show through at times, however, revealing him to be a kind and loving father. At the end of 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.