Lion-O (voiced by Larry Kenney in the original series, Will Friedle in the 2011 series) is the leader and the hereditary "Lord of the Thundercats".  He is the main character in the television series franchise "Thundercats". Lion-O, based on the lion, wields the legendary Sword of Omens, which is able to fire bolts of energy and allows Lion-O to see across great distances with its power of "Sight Beyond Sight", as well as the Claw Shield, a gauntlet that launches grappling lines from its claws. A mere child of twelve years old at the time of Thundera's destruction, Lion-O aged to adulthood during the trip to Third Earth when his suspension capsule failed to prevent him from aging too much. 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.

In the latter half of the show's first season, Lion-O has to put all that he has learned to use in the "Anointment Trials", which consist of contests of strength, speed, cunning, and intelligence (this last is referred to as "mind-power" in the story arc) against each of the other Thundercats. (Complicating matters for Lion-O is the fact that he is required to be unarmed for the contests; moreover, the other Thundercats are not permitted to assist him. The Mutants attempt, but ultimately without success, to take advantage of this last in an effort to leave the Thundercats leaderless). Ultimately triumphing over Mumm-Ra in a final battle ("The Trial Of Evil") in the heart of the villain's pyramid in which he discovers that Mumm-Ra, like any Egyptian mummy, is dependent upon his sarcophagus, Lion-O is crowned the true Lord of the Thundercats in a grand inauguralceremony attended by nearly every inhabitant of Third Earth. It is also hinted that he has romantic feelings for Willa, and Mandora.

In the 2011 version, Lion-O is a teenager instead of being a boy inside an adult body. Lion-O can also be a bit of a hothead but is by far the most patient and understanding cat next to Cheetara. He is the only one at the beginning who believes that technology and Mumm-Ra exist even though technology is sold across Third Earth. Lion-O believes that the other races of Third Earth should be treated equal as seen when Lion-O was defending two persecuted Lizards from some Thunderians until Claudus broke up the fight. After Claudus was killed by Mumm-Ra, Lion-O becomes the new Lord of the Thundercats. His catchphrase is "Whiskers!" whenever he finds himself in a bad situation. In "Legacy", it is revealed that Lion-O had an ancestor named Leo who played a part in the defeat of Mumm-Ra. Lion-O's weapons are the Sword of Omens and the Claw Shield. It is shown that Lion-O had feelings for Cheetara until in "Between Brothers" when she confessed her feelings for Tygra. In "Native Son", it is shown that Lion-O had an unnamed mother who died giving birth to him. In Recipe for Disaster it is shown that Lion-O had attempted to work through his relationship issues with Cheetara by pursuing Pumyra, trying multiple times to court her, all of which ended in disaster. However, after saving her from Mumm-Ra yet again she gives him a kiss on the cheek in thanks. However, in "What Lies Above Part 2", the feeling was revealed to be an act as she reveals her loyalty and love to Mumm-Ra and only toyed with Lion-O's emotions to further her advantage.

Lion-O is renamed León-O in the Spanish version, Leo in the German version, Starlion in the French version, and Lion in the Brazilian version. Originally Lion-O was to be named Lion-L in the English language version.

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