Lion-O was a brat. The youngest of the ThunderCubs, he was the heir to the title Lord of the ThunderCats. He didn't exactly endear himself to the crew of survivors of Thundera at the beginning, but they all knew how important it was to protect him. When a group of Mutants (giant, anthromorphic beings more animallike in appearance than the feral ThunderCats) invaded the ship, Lion-O wielded the Sword of Omens, a mystical weapon that belonged to the once-and-future Lord of the ThunderCats. He picked it up, realizing that he was indeed the chosen one.

When the survivors arrived to their destination, there weren't many changes to those that survived. Jaga, Lion-O's mentor, died so the others could live, and Lion-O grew to maturity. Well, he was physically changed, but mentally, he was still a child.

He proved himself against all obstacles throughout his life on Third Earth. However, it was Lion-O's trail by fire that helped cement his place as the rightful Lord of the ThunderCats. Although he earned that title by birth, he didn't earn it by tradition. The Trials had Lion-O going solo against his fellow Thundercats in five days without his sword and claw shield. He had to be stronger than Panthro, craftier than Wily Kit and Wily Kat, faster than Cheetara, and stealthier than Tygra, all of which he had completed. These challenges took place within four days, but the fifth day was the hardest challenge of them all. Lion-O had to defeat his worst enemy, the benevolent Mumm-Ra The Everliving. Using all the skills he acquired throughout his battles and the confidence he earned during the challenges, Lion-O defeated Mumm-Ra and rightfully ascended to the title that had always been his birthright, Lord of the ThunderCats, and, in a way, matured to become an even greater hero than he was before.

Lion-O proved to himself, his compatriots, and the viewers that he was the definition of a hero. Not perfect by any standards, but willing to protect anyone from any danger that looms their way. He's Toonami's first hero.