Leo had a slender build, much like Lion-O's. His hair and sideburns were much longer, suggesting he was older than Lion-O at the time he led the animals' rebellion against Mumm-Ra. He wears blue-gray armor with the Thunderian red symbol in the center. However, he has also been seen in golden armor after activating the claw shield.
Leo was a powerful and agile warrior. He was a capable swordsman, and was able to utilize the power of the Armor of Omens with great skill.
He was able to rally the various species under Mumm-Ra's command, suggesting a great talent for persuasion. As stated by Mumm-Ra, he was a remarkable tactician.
Leo was much like Lion-O in the fact that he excelled in battle. He shared many of Lion-O's qualities. He was less hot-headed then Lion-O, but had his moments. He and Panthera seemed to be dating, although it was never truly confirmed. He was brave in battle and caring for the animals around him. Although in Birth of the Blades, it is revealed Leo was very loyal to Mumm-Ra, under the delusion that Mumm-Ra was spreading order throughout the galaxy. Mumm-Ra himself deemed him loyal enough to deserve a planet of his own to rule (under Mumm-Ra's supervision, that is). Panthera's words against Mumm-Ra made Leo uncomfortable, as he was told such talk was treason. But when Mumm-Ra struck Panthera with energy for speaking out of turn and witnessed the destruction of a solar system that supported three life-bearing planets, he finally realized Panthera was right, and was the one who came up with the idea of forging the Sword of Omens.
Leo led the rebellion against Mumm-Ra because Mumm-Ra was on the verge of obtaining the War Stone. With the stone in his possession, Mumm-Ra would become virtually invincible. Leo was by Mumm-Ra's side when he learned of the theft of the mysterious red gem, by Tygus, possibly the ancestor of Tygra. Knowing Mumm-Ra's plan to put it into the Sword of Plun-Darr, Leo took the stone from Tygus, joined it with his own newly-forged Sword of Omens, and used it to defeat Mumm-Ra.
Leo is the first Lord of the Thundercats and founder of the city-state of Thundera.
But once Mumm-Ra was defeated, and the animals settled upon Third Earth, it was decided the power stones were too dangerous to stay in the possession of one species, and it was agreed they were to be divided.
Leo and the Cats kept the War Stone, which had since become the Eye of Thundera, and an irreplacable part of the Sword of Omens.
- In the episode Legacy, Lion-O experiences what Leo did, in order to access the Book of Omens.
- In Birth of the Blades Leo is voiced by Jason Marsden, who is a close personal friend of Will Friedle.
- Leo was briefly seen in What Lies Above, Part 1 when Vultaire was telling the ThunderCats how the four Power Stones were divided among the animal races after landing on Third Earth.