The hereditary Lord of the ThunderCats, Lion-O is the son of Claudus, the ruler of Thundera at the time of its destruction. Initially a boy stuck in a man's body, the years saw Lion-O develop the wisdom and skills needed to lead the ThunderCats on Third Earth, and later lead the rebuilding of his home world of New Thundera. Lion-O can be quick to anger, and is impulsive and headstrong, but also possesses great moral values, at the forefront of those being the Code of Thundera - Justice, Truth, Honour, and Loyalty. Lion-O is fierce defender of freedom and a brave and noble leader, one that the other ThunderCats are proud to stand beside.

Exodus

Unaware of the chaos going on around him, The young Lion-O slept peacefully aboard the Thunderian flagship which flew away from his dying home planet of Thundera. He was awakened by Cheetara, and despite his nursemaid, Snarfs, objections, he went to the bridge at the request of Jaga. There he witnessed the destruction of his home, and was shown the mystic Eye of Thundera by Jaga, as well as the Sword of Omens. When the Mutants attacked the ship, Lion-O remained in the sword chamber, where he was eventually found by S-S-Slythe and Jackalman, Lion-O barely had the strength to lift the sword of omens, but he eventually managed to, and the eye of thundera saved the Thundercats by driving the Mutants from the ship.

Lion-O then sadly bid farewell to Jaga as the Thundercats entered their suspension capsules. When the ship crashed on the designated planet, Lion-O's capsule was flung from the ship, and he amazed to see that he had aged dramatically. Lion-O believed he was no longer a boy, and batted away the toys offered to him by Snarf. He later awakened the other Thundercats, who were able to fight back the Mutants.


The Unholy Alliance

Although the young Lion-O had aged in the capsule, he had missed out on the life experiences that made a person a man. Lion-O went into the woods, scouting the surrounding area where the ship had crashed, Lion-O came up against the Mutants, and defeated them with the aid of the other Thundercats. He chose to go on alone and look for Tygra and Cheetara, this time encountering the evil Mumm-Ra the everliving, Mumm-Ra would have destroyed Lion-O had he not caught sight of his own reflection and fled.


Berbils

While out with Snarf, Lion-O was captured by the Ro Bear Berbils. Although initially believing them to be hostile, and mocking him, he soon learned they were peaceful creatures, and defended their village from a swarm of locusts sent by Mumm-Ra.


The Slaves of Castle Plundarr

Lion-O immediately raced to the aid of the primitive Brute Men, who were being used as slaves by the Mutants. Lion-O knew he could not harm the Brute men, but refused to move when they advanced on him after a dose of warp gas. They snapped out of their trance just in time, as Cheetara told Lion-O that his pride, carried too far, would be foolishness.