Leo was a character in the 2011 Thundercats television series. Leo was a Cat who became the first Lord of the Thundercats, and he was also a distant ancestor of Lion-O.
As a young man Leo was a servant of the undead sorcerer Mumm-Ra. He eventually rose to become the head of Mumm-Ra's armies. As such he led many campaigns on many worlds, conquering many worlds while helping Mumm-Ra search for the War Stones. Leo was friends with Captain Tygus and Panthera.
Leo initially fully supported what Mumm-Ra was doing, feeling that Mumm-Ra represented the best chance to impose order on the galaxy. In return for his service Mumm-Ra stated that he would give Leo a planet of his own to rule, under Mumm-Ra's supervision of course.
Panthera soon approached Leo with her concerns about Mumm-Ra. Leo initially brushed off her arguments as being treasonous talk. However after seeing the lengths to which Mumm-Ra would go to achieve his goals he began to have his doubts. Leo was horrified to see Mumm-Ra attack Panthera when she argued that they could not destroy the inhabited Plunn-Darr system. Leo tried to convince Mumm-Ra not to attack Plunn-Darr, but the undead sorcerer's mind was made up. Mumm-Ra had Leo destroy the Plunn-Darr system - at the cost of billions of innocent lives - to create the material which he could forge the Sword of Plunn-Darr with.
After that and the news that the final War Stone was located, Leo decided at that point to act. He was able to gather the scraps of the material used to create the Sword of Plunn-Darr and had a blacksmith create the Sword of Omens. Leo then took the recovered war stone from Captain Tygus and placed it in the Sword of Omens, whereupon it transformed into the Eye of Thundera.
Leo confronted Mumm-Ra and was able to defeat the ancient mummy in battle. Leo led the other slaves on Mumm-Ra's ship in revolt against him. During the fighting Mumm-Ra sealed himself in his sarcophagus and his ship crashed on Third Earth.
Following the crash Leo became the first Lord of the Thundercats. The Power Stones were split up amongst the various species as it was too dangerous to leave them in the hands of any one species. Leo would go on to have a family, and his descendants ruled the nation of Thundera. Leo eventually became the ancestor of King Claudus and his son Lion-O.