Rainuma was a middle-aged man with a wrinkled yet chiselled face, finely brushed black hair and a goatee. He is usually seen smoking a pipe as well as a business suit and lab clothes.
Rainuma is a devoted scientist who believes in human progress, having previously won the Nobel Prize for his studies. He becomes bitter after witnessing his friends and family being slaughtered by demons.
Subsequently he dedicates all of his research to finding out a way to destroy demons altogether, and coldly rebukes requests of mercy from his test subjects like Mico, who contrary to his views was a Devilman and not a demon. This difference makes very little difference to him though and will even have anyone who fits his quota as a demon killed. Perhaps not a morally evil man, but his actions caused the deaths of thousands of innocents and greatly contributed to the downfall of human society.
Dr. Rainuma is first seen in the Center of Biological Studies as he and his fellow scientists conducted torturous experiments on the young Devilman Mico who was encased and bound in a small chamber, electrocuting her until she sprays her acids in pain. Rainuma tells his assistant to examine the said acids before he departs to an operating room to talk to a doctor. The doctor tells Rainuma his discoveries from examining several deceased demons and their biological tissue was wildly different from anything they had encountered before.
Later on Rainuma studies in his lab and looks over his results from the demons he had experimented on and the bodies of those who had died in the earlier mass suicide fusion run. He comes to a conclusion that anyone who showed any anxiety, stress, or delusion was indeed a demon. He calls in a mass press interview to announce his discoveries across the world, further cementing the recently introduced Demon Busters.
After Akira Fudo is informed of the unfair and painful experiments performed upon his fellow Devilmen in Hokkaido at the Centre he goes there and finds Rainuma. He grabs him and forces him to lead him to his test subjects. However when he arrives he only finds Mico still alive. He doesn't appear after this in the original manga and was assumed to have been killed.
- Rainuma's design was used as a army commander in the Mazinkaizer vs. The Great General of Darkness OVA.
- While never given a first name in his inital appearance, he is given one in other spin offs like 'Devilman Grimoire' and 'Amon: The Darkside of the Devilman'