Sumu is a large black scorpion with four gleaming red eyes and a soft gray triangular marking on his forehead. He has other gray markings all over his body, including three gray spots on a pair of very large pincers. His tail is tipped with a sharp stinger that is covered with a dangerous venom; the only known antidote is ash from the Outland volcano.
Described to be a highly dangerous and deadly killer, Sumu will go to any lengths of killing his victim(s) with a venomous sting. Fierce and loyal to Scar, Sumu will follow his commands without hesitation, though is not above feeling anxious about fighting, as seen when he learns that Scar plans to bring the Lion Guard into the Outlands Volcano, proving that he is a careful attacker. Sumu is generally quiet and prefers to let others speak.
Scar has recruited Sumu to sting Simba on the anniversary of Scar's defeat. At first, it seemed like he was as large as a lion, but as the camera humorously zoomed out, he is revealed to be an average-sized scorpion. Despite being much larger, Janja is terrified of him, jumping onto Reirei and Kiburi to avoid his venom, with only Ushari fully aware of the role that he will be playing. After informing the leaders of Sumu's exact role in the plan, Scar sends Sumu out to inject Simba with his venom.
Sumu is later seen climbing Pride Rock while Rafiki and the Pridelanders sing, Good King Simba for the Kumbuka celebration. During the song, Sumu seizes the opportunity to sting Simba on the tail, poisoning him. The diminutive arachnid then makes a dash for the Outlands, where he is later spotted by Ono who suspects him to be the cause of Simba's predicament. Kion then leads The Lion Guard after him. The young lion pounces on the scorpion when close. But when it seems like he's gotten him, Kion notices the scorpion has escaped down a hole. Ono then investigates above. He sees Sumu escaping out the other side of the tunnel, which leads back into the Outlands.
After returning to the Outlands Volcano, Scar praises Sumu for his work. The evil ghost then commands Ushari to be ready, as the Lion Guard will be arriving for the volcanic ash to heal Simba, to which the evil snake agrees to do. Sumu then shows confusion, questioning Scar on why he'd want the Lion Guard to come to him. Scar then informs Sumu that he has helped him to set the perfect trap, before laughing ominously.
He is one of the show's antagonists that has a limited amount of screen time.
He is the first character in the show who is an invertebrate, as well as the first invertebrate to speak.
Sumu is the Tanzanian red clawed scorpion. He mostly resembles an emperor scorpion, but that species doesn't have deadly venom like him.
He is the third venomous villain in the show, the other two being Ushari and Kenge.