Like his brothers, Leonardo began life as a turtle hatchling who was accidentally doused in a mutagenic ooze, which gave him a more human-like body and mind.
At the story's beginning, Leonardo led his brothers in the rescue of April O'Neil from a band of thieves who were assaulting her. Leonardo was extremely excited about fighting his first battle, and eagerly told Splinter of their accomplishment, though he attempted to be somber when his father was speaking.
He was intensely dismayed when Raphael brought the unconscious April O'Neil to the lair, but did not protest further when Splinter gave his approval. He stood by while Splinter explained their origin to April, and helped escort her back to her home. After an evening of pizza and jokes, Leonardo insisted on returning home because Splinter would worry. However, they soon discovered that Splinter had been kidnapped and the lair ransacked, devastating the Turtles.
Leonardo's cautious, tactical approach to finding Splinter soon brought him into conflict with Raphael, who wanted to be more proactive in their search. After a heated exchange, Raphael stormed up to the rooftop, where he was savagely beaten by a group of Foot ninja and thrown through the skylight. Leonardo led Michelangelo and Donatello in the battle against the enemy ninja, but they were driven into retreat.
At April's country farmhouse, a guilt-ridden Leonardo kept a constant vigil over the comatose Raphael. When Raphael awoke, Leonardo was overjoyed and apologized for his previous harsh words, and the brothers were reconciled. As Raphael recovered, Leonardo and his brothers spent many days training, but Leonardo was still deeply troubled by the loss of their master.
While he was meditating in the woods, he experienced a psychic connection to Splinter. That night, he led his brothers in meditation, allowing them to briefly contact their father. The following morning, they decided to depart for New York again, in order to free him.
On their first night back, they were attacked by the Foot Clan, a battle that culminated in the Shredder appearing before them. Leonardo fared the best among his brothers, wounding the Shredder's arm with one of his swords, but ended up pinned down at the point of Shredder's spear. Fortunately, Splinter reappeared and defeated Shredder, throwing him from the rooftop into a garbage truck.
Along with his brothers, Leonardo warmly embraced their father, before celebrating their victory.
In the first film, Leonardo was fairly modest and sensitive, rarely issuing direct commands and seemingly much more relaxed around his brothers thinking of himself as more of an equal than a leader; he also joked around with his brothers much more than in other versions of the TMNT. Since the film was mostly based on the Mirage comics, one can best compare Leonardo's personality with his counterpart from the comics. He is rather quiet, always trying to put together a plan before he attacks and is again the one that is closest to Splinter.
In the second film, Leonardo was much more prominent and his leader position was brought to focus. He is seen on many occasions bickering with Raphael as their sibling rivalry begins to become much more serious. He, like his brothers, was astonished at the return of the Foot but he found that their current homelessness since their last battle was a more pressing issue and soon he convinced his brothers that they needed to move. Leonardo is once again sensitive, caring, and humorous in this adaptation but he now displays far more aggressive leadership tendencies.
In the third film, Leonardo helps his brothers back in feudal Japan to go back home after helping a village in trouble from the greedy-western-colony driven man, Walker. Leonardo at this point in the movie series had become less and less of a leader, shouting out wise cracks and goofing off with his brothers, a flaw not present in other incarnations of Leonardo. He was the one who defeated Lord Norigana by trapping him within a Japanese Bell before cutting his pony tail off mocking him about a new haircut. Like his brothers minus Donatello, he seemed at home in ancient Japan. In this film there wasn't much for him to really do, he was just portrayed as a goofball making only a few smart decisions.
In the fourth film, Leonardo has been away for a long time, after being sent on a journey by Master Splinter to hone his skills in becoming a more efficient leader after the Shredder's defeat. April finds him in Central America, and once again, Leonardo's desire for perfection was shown, as even as his real training was over for a year, he didn't want to return because he feared he may still not be good enough. While he was hesitant to return to New York City, he does at the right time to take on the dangerous new force of evil.
His relationship with Raphael is strained as Raphael felt abandoned by Leonardo as well as feeling less appreciated by Splinter. Leonardo's vision of the world is perhaps wider than Raphael's; in the first movie prequel comic, Leonardo becomes angry with Raphael for trying to leave them in order to save a man from being mugged, because there are four heavily armed Triceratons in the sewers who could cause devastation to the city. He becomes further angered when Raphael deserts them mid-battle to help the old man. This conflict suggests that the two brothers operate on different levels of morality, though neither is necessarily wrong. Raphael states in the comic that he was tired of waiting for disaster to fall on his family, and tired of fighting aliens while people in their own neighborhood are being mugged and murdered. Leonardo, on the other hand, believes that the world of men is the responsibility of the police, while Utroms and Triceratons are their domain..., and that they should fight only when there is no one else to solve the problem. This also engages Leo in a contradiction when he stays in Central America, using violence to fight local lawlessness and effectively deserting his brothers because he believes, as Raphael believes, that others need him more. Such parallels suggest that the two brothers are experiencing the same dedication to justice but in different mentality, albeit in very different locales and using different tactics.
In the TMNT CGI movie, Raphael challenges Leonardo after arguing of their own individual sense of justice and the reasons for their actions. Leonardo also discovers that Raphael is the Nightwatcher and they engage in an emotional fight. He is defeated when Raphael breaks his swords and pins him to the floor. Raphael almost kills Leonardo out of anger, then retreats in horror of his own actions because of shame, remorse for his actions, and his brother's deep and confused stare. Leonardo is captured by the Stone Generals and Foot Clan, but is rescued by his family later before the final battle where Leonardo and Raphael finally resolve their differences. Raphael accepts Leonardo as their leader while Leonardo confesses to needing Raphael. In the end he realized that he can do anything if he has his brothers by his side.
- According to the T files on the DVD extras of the 1990 film, Leonardo's:
- Hobbies are collecting candles, if it counts.
- Favorite music is Traditional Japanese, and New Age (for meditating).
- Favorite foods are pizza, rice, fish, sushi, and apples.
- Favorite book is On the Art of War by Sun Tzu.
- Favorite color is blue.
- Is right-handed.
- Height is 5'1.
- Weight is 170 lbs.
- Eyes are brown.