Lion-O is the son of Claudus, and is the leader and hereditary "Lord of the ThunderCats". His adventures are shown from childhood to adulthood onThird Earth and New Thundera, together with his friends and followers, the ThunderCats.
At the beginning of the series, Lion-O is a boy of mere twelve years old. He was among the group of Thunderian nobles to make the trip to Third Earth in the flag ship of the Thundercats, when their planet Thundera was destroyed. During this journey he aged physically until he became an adult of age twenty-four; this was due to the suspension capsule diminishing but not altogether preventing aging ("Exodus"). Coincidentally, such a malfunction did not occur with Wilykat and Wilykit, who were around the same age as himself, though as Lion-O recalls in the series' first episode, his mentor Jaga had mentioned that some aging does take place while in the Suspension Capsules in which he and the other ThunderCats were placed when their flagship was damaged.
The now grown-up Lion-O assumed that this was the reason for his aging to adulthood, though he did not express any confusion regarding why WilyKat and WilyKit had not also aged as he had, which seems to indicate that he simply accepted it as a quirk of the capsules. Although cunning and skillful, he is truly a child in a man's body, and throughout the series, must learn what it takes to become a true leader and gain true maturity.
