Nimnul has receding red hair, wears very thick glasses and has a high-pitched laugh.
Though Nimnul is an intelligent and creative scientist, his plans are often illogical and convoluted. For example, he kidnapped all the cats in the city to make an immense amount of static electricity and in his appearance in the pilot, he constructed a laser cannon designed to create a giant gelatin mold which would be used to cause an earthquake under the United States Gold Reserve. And on another occasion he kidnapped all the mice in the city to harness the power of electricity to bake all the potato plants in Idaho. Aside from his scientific genius and convoluted eccentricity, Nimnul is an egotistical, greedy and evil madman whose goals often range from becoming a wealthy criminal genius to taking over the world and ruling with an iron fist. As such he is uncaring and inconsiderate about the safety of others and does not care what damage his inventions do.
Despite these malicious tendencies, he has tried to go legit on a few occasions, however during presentations, his inventions always backfire or get laughed at which results in him losing his temper and going back to his criminal lifestyle. Nimnul suffers from chiroptophobia (aka a deep fear of bats) as he believes that they will try to suck out the blood from his neck. He also has a deep liking for prunes and they appear to be his preferred food.
Nimnul was first introduced in "To the Rescue", where he is a mad scientist working for Aldrin Klordane. However after Klordane was defeated by the Rescue Rangers, Nimnul continued working on his own.
In "Catteries Not Included", Nimnul invents a device for a science expo which can harness static electricity from cats and turn it into power for the whole city, however his creation was quickly laughed at by his peers which made Nimnul enraged and vengeful. He then concocted a plan to kidnap all the cats in the city using a team of Robot Dogs and then use the cats to power his static electricity doomsday weapon in order to use it to threaten the city into giving into his demands. Eventually, the Rescue Rangers discovered his plans after discovering his Robot Dogs, and were soon able to free the cats and stop Nimnul's evil plan.
In "The Pied Piper Power Play", after Nimnul's discovery of instantly turning potato plants into baked potatoes using only pure electricity was laughed at during a science convention, he decided to turn all of Idaho's potato plants into baked potatoes as revenge, however when his electricity gets cut off he invents a device to hypnotize all the mice in the city in order to have them generate electricity for him. This resulted in both Monterey Jack and Gadget Hackwrench being hypnotized as well, luckily Chip and Dale were able to locate them and free all the imprisoned mice and destroy Nimnul's lab after sabotaging his machine.
In "Rest Home Rangers", he tried to turn over a new leaf and become a legitimate inventor when he made a prune-powered aging device to turn milk into cheese in an instant, however when spectators where disgusted by his invention when it failed to age milk into cheese and only aged it into sour milk, his anger and vindictiveness got the better of him and he swore to use his aging device on the whole world. After his device unintentionally hits Monterey, the Rescue Rangers become determined to stop him and find a way to restore Monterey to his younger self. After confronting Nimnul at the prunes factory, they are able to use plums to reverse the effects of the aging device and restore Monterey to normal as well as get Nimnul arrested. During this incident, it appears that Nimnul has finally familiarized himself with the Rescue Rangers, as he realized that every time these same "rodents" show up, his plans go awry.
In "A Fly in the Ointment", Nimnul invents a device that allows him to travel through phone lines which he uses to instantly transport himself inside buildings and steal all their valuables. However when his device suffers a malfunction while trying to get rid of Zipper, the device ends up switching their bodies while still retaining their original heads (which have changed size to match their new bodies). It is during this mutation that he finally gets to formally interact with the Rescue Rangers and discover that they can speak, much to his shock. He then tells them of what occurred and they agree to help him in order to restore Zipper as well, but when Zipper is taken by the army, who assume that he is an alien, the Rangers and Nimnul have to find a way to save him. The group then use Nimnul's device to teleport into the army base that is holding Zipper, however the device malfunctions again, causing the rangers to switch bodies as well. The group then heads out to find Zipper while Nimnul and Gadget repair the device. It is during this time that Nimnul becomes aware of Gadget's high intelligence, which he finds impressive despite her being a mouse, much to her offense. Once Zipper is brought back to them, Nimnul reveals that his unable to repair the device due to a damaged part, however Gadget is able to repair the device with a mere paperclip, thus upstaging the surprised and annoyed Nimnul. The group then use the repaired device and teleport back to Nimnul's lab where they re-appear with their normal bodies restored to normal. However Nimnul does not waste any time in capturing the Rangers, regardless of their brief truce and alliance together, and reveals his plan to destroy them as revenge for their past actions against him. Luckily, Zipper is able to reactivate Nimnul's device which ends up transporting him straight to the police.
Nimnul has appeared many times within the Rescue Rangers comics.
In the Boom! Studios revival comics of the Rescue Rangers, Nimnul is shown to still antagonize the city and continues to be thwarted by the Rescue Rangers.