Leonardo is one of the four protagonists of the Teenage Mutant Ninja Turtles. He is the eldest brother of the TMNT and has a special relationship with Karai, the relative of his archnemesis the Shredder.
He never explicitly referred to himself as leader in the early stories, except in issue #44 ("The Violent Underground"). He is the one to usually take charge of the turtles when Master Splinter is not present. He is often at odds with his more hot-headed brother Raphael.
In Leonardo #1, Leonardo goes out for a run on the rooftops of New York City and is ambushed by the Foot Clan. He puts up an admirable fight against an army of Foot Ninja, but is eventually overwhelmed. Beaten to near unconsciousness, he is thrown through April O Neil's apartment window. The remaining Turtles and Splinter are forced to continue the fight, but even with the aid of Casey, the odds are against them. In the end, the building catches fire and the police arrive, but they secretly escape to Northampton. During this time, Leonardo recovers from his physical wounds. However, he lost a great deal of confidence. He repeatedly attempts, unsuccessfully, to hunt for deer. While out hunting, he sees April fall through ice into a lake, and he rescues her. In subsequent issues, it is implied that Leonardo has regained most of his confidence.
In the Return to New York storyline (Teenage Mutant Ninja Turtles (vol. 1) #19-21), Raphael demands that the Turtles return to New York to confront the Foot Clan and the Shredder. He accuses Leonardo of cowardice, and the arguing brothers soon come to blows. Leonardo is beaten by Raphael, who throws Leonardo through the wall of the barn and leaves alone. Along with his younger brother Donnatello and his youngest brother Michaellanglo, Leonardo returns to New York and reunites with his wayward brother in the old sewer lair. The three go along with Raphael's plan to storm the Foot Headquarters, where once again Raphael goes off on his own to fight the Shredder. However, he is ambushed and beaten by the Shredder's Elite guard, but is rescued by Leonardo. This prompts Raphael to finally cede to Leonardo's leadership, leaving him to fight the Shredder. Leonardo engages in a bloody battle with Shredder that spills out onto the rooftop of the building. Leonardo ends the battle by decapitating the Shredder just as the building implodes. The Turtles later burn the Shredder's corpse in a funeral pyre in a nearby Manhattan harbor.
In the City at War storyline (Teenage Mutant Ninja Turtles (vol. 1) #50-62), a feud between various factions of the Foot Clan over leadership breaks out. As fighting ensues in the streets of New York, the Turtles and the civilian populace get caught in the middle. Leonardo grows weary of constant battle with the Foot Clan and seems fraught with indecision. The Turtles are approached by Karai the leader of the Foot Clan in Japan who has come to New York to unite the Foot. She presents the Turtles with an offer of a truce between the Foot and the Turtles if they help her kill Shredder's Elite Guards, which are a major obstacle to her reorganizing the Foot. Despite Raphael's objections, Leonardo persuades his brothers to accept Karai's offer and all four Turtles successfully work with Karai to eliminate the Elite Guard.
In Volume 2 of the Mirage Stuidos comic, the turtles begin living in separate places. Leonardo decides to live in a newfound sewer lair. Michelangelo and Raphael notice a change in Leonardo and note that he seems more easygoing, though Raphael points out that his and Leonardo's natural order is to be "buttin' heads."
Years later in Volume 4, Leonardo still leads his brothers (all four now in their thirties) in fights against crime. Leonardo and Raphael's conflict seems to have greatly lessened. When the Utroms make a very public arrival on Earth and reveal alien life to humans, however, the Turtles become free to mingle in everyday society. The Turtles also help the Utroms acclimate to life on Earth and work alongside the Foot Clan as security. One Foot Clan member is Cha Ocho, who Leonardo has a rivalry with due to an encounter years earlier. Karai approaches Leonardo for help when a mysterious force begins attacking various Foot Clans; only the New York branch is left intact. His investigation takes him to the Battle Nexus, where he meets Oroku Yoshi (who wears armor similar to the Shredder's)..
In Volume 3 of the I mage Comics series, Leonardo was initially portrayed as similar to his Mirage counterpart (at the time, Image was picking up where Volume 2 left off). In the later issues, he lost a hand when it was eaten by King Komodo, although this did not seem to deter him significantly. He tried initially to use a prosthetic hand, which was given to him by Donatello, but he much preferred to wear a steel cap which came with a retractable blade.
The Archie Comics series initially began as an adaptation of 1987 animated series, so Leonardo was naturally portrayed like his animated counterpart. As the series progressed, it began telling original stories. Leonardo demonstrated a rather strong dislike for firearms. Also, a future version of Leonardo was depicted, having founded a ninja school. Four of his top students were depicted: Nobuko, possibly his love interest; Miles, a young black man; Carmen, a Latina woman and possibly his love interest; and Bob, an anthropomorphic baboon. These students seemed to have an "extended family" relationship with the Turtles, Bob in particular referring to them as uncles.
Leonardo appears in the IDW series as the leader once more.