Emperor Gruumm is the figurehead leader of the Troobian Empire and the main, later secondary villain of Power Rangers SPD.
Gruumm led the Troobian Empire in many of its conquests, overwhelming each planet with large-scale assaults and then draining it of its resources.
However, his most notable conquest was the planet, Sirius, where he faced off against the first incarnation of S.P.D. Gruumm personally battled Anubis Cruger, who cut off one of his horns, causing him to develop a grudge against Cruger.
In year 2025, he began his attempts to take over Earth and succeeded in turning the S.P.D. A-Squad Rangers to his side.Gruumm first sent a Blue-Head Krybot to obtain a Proton Accelerator, but this failed due to interference by S.P.D. He continued to launch assaults on the Earth, aided by Mora, a young girl who could create monsters from drawings, and Broodwing, an alien weapons dealer, who provided him with Krybots, weapons, and giant robots. Gruumm later transformed Mora back into her true self, Morgana, when she had failed him too many times. He eventually decided to go to Earth himself and challenged Anubis to battle in order to settle the rivalry they started on Sirius. The B-Squad battled Gruumm, only to be unable to match his combat skills and motor bike and later Supreme Commander Birdie Fowler faced him after he had discharged Cruger. The emperor easily beat him as well until Cruger arrived, morphed into the Shadow Ranger and managed to drive him off. Later, Gruumm became tired of paying Broodwing for weapons and robots that kept failing anyways and forced him to provide his services for free. However this made Broodwing set out to conquer the Earth first as a form of payback. Some time after this, Gruumm used a wormhole to go to the past to take over the Earth before S.P.D. ever existed and recruited Zeltrax in helping him take over. But, even their combined forces were unable to match the combined forces of the S.P.D. Rangers and the Dino Thunder Rangers and though Gruumm planned to stay in the past, he headed back to 2025, when Mora reminded him that he would have to re-conquer all the planets he had taken control of in the future. As time went by, it became clear that Gruumm had been working on a long-term plan since he had not been employing his usual strategy. He hadn't been working to leech from and destroy Earth and had instead stealing natural resources from the planet such as diamonds, gold, Iridium and Hemotech Synthetic Plasma. Gruumm planned to use these items to construct a body for the being, Omni, who was the true leader of the Troobian Empire. Later, when Omni launched his final assault on Earth, Cruger, who had been captured, escaped his cell on Gruumm's ship and the two engaged each other. However, after Omni's defeat, the ship was destroyed as well, though both Cruger and the Emperor survived. Gruumm challenged his nemesis in one final battle, only to lose and have his other horn cut off before being imprisoned.
Gruumm is a mighty warrior, the strongest the Troobian Empire has to offer, which is part of why he is its supposed leader. He wields a staff weapon that he can fire energy blasts out of and can shape-shift and use telekinesis, along with a limited form of telepathy seen when Gruumm spoke with Omni. He also rides a powerful motorcycle that can fire lasers.
- Gruumm is the main villain of the Nintendo DS version of Power Rangers: Super Legends, while in the console version, he is only mentioned briefly.
- Before Omni actually appeared, Gruumm would often talk with him on his throne, but was seemingly talking to himself since though he spoke out loud, their conversations seemed to be telepathic in nature and Omni's responses were never heard.
- Gruumm is an example of a main Power Rangers villain who has no Super Sentai counterpart. Other examples are Ransik, Lord Zedd, and Mesogog.