Leonardo Splinterson is the main protagonist of the 2003 Teenage Mutant Ninja Turtles cartoon series. He is the eldest brother of Donatello, Michelangelo, Raphael, the eldest adoptive son of Splinter, and the leader of the Turtles.
Leonardo has the same appearance as his brothers, except he wears a blue bandana and his signature weapons are two katanas.
Leonardo is the leader with honor, calm, and reassurance. He’s most "spiritual" of the four, and kind of a grown-up, psychologically because of all of his seriousness. He has a very close bond with Splinter, a strong sense of honor and ethics, along with being a dedicated follower of the Bushido code. He is usually the one who fights the Shredder the most and save the day. He is also determined to believe in the best of people, even potential enemies. Because of all those qualities, Leonardo is viewed by his brothers as a pillar of strength.
Though mature calm and level-headed as he may seem, Leonardo can easily gets angry just like Raphael, if not even worse. The two also occasionally argue, although they nevertheless get along well. Leonardo is shown to be darker and more traumatized briefly in season one when losing his swords, and for the first half of season four due to unable to get over his failure, to the point of injuring Splinter out of rage. However, he later regains his confidence after being trained with the Ancient One.
Like his brothers, Leonardo was once an ordinary pet turtle bought by a young boy, until an accident caused the boy to drop them all into the sewer. The mutagen from the truck in the accident dropped into the sewer as well, covered them and the passing-by Splinter who adopted them afterwards. The mutagen mutated them all into being anthropomorphic. Splinter would ever since train them ninjutsu to protect them from the outside world.
Teenage Mutant Ninja Turtles
Leonardo and his brothers would then fight criminals and thwart their plans in the night. During this time, they also befriend April O'Neil and Casey Jones. However, their effort attracts the attention of Oroku Saki, the leader of the Foot Clan and the mastermind behind those crimes. He invites Leonardo to join him, but Splinter then tells the Turtles about Saki's true identity, the Shredder, thus Leonardo refuses. Because of this, the Turtles become the Shredder's archenemies, especially after the reveal of his true identity as the Utrom criminal Ch'rell, who caused the Utrom spaceship to crash on Earth a millennium ago, thus set up a chain of events that led to the turtles and Splinter's mutation. Later on, Leonardo also has a love-hate relationship with the Shredder's adoptive daughter and second-in-command, Karai, constantly convincing her about the Shredder's evilness, but she's unable to leave him due to how much he has provided her since adopting her from the streets.
Through much of the fourth season, while the other Turtles are fully healed and recovered from their battle with the Shredder, Leonardo still can not get over his failure and becomes more and more aggressive until he injures Master Splinter out of rage. Ultimately, Splinter sends him to train with the Ancient One, and he returns when Karai, now the new Shredder, seemingly kills his family. Thankfully, they all survive and find a new lair. Leonardo then have a fight with Karai, before telling her that he gives her one last chance to leave his family alone.
In the fifth season, Leonardo and his brothers become the Ninja Tribunal's acolytes and thus he doesn't receive a weapon from the Spirit Forge, likely because it senses a greater force in him. He and his brothers train themselves to fight against the Tengu Shredder, of whom the Utrom Shredder stole identity from. In the Fast Forward season, he and his brothers alongside Splinter are accidentally transported to a hundred years in the future, befriending Cody Jones, Casey and April's descendant, and Serling, while battling evil forces like Cody's uncle Darius Dun, Sk'Okanabo and Viral. They return to the present in the Back to the Sewer season and fight against a cybernetic copy of the Utrom Shredder, dubbed the Cyber Shredder. They then attend Casey and April's wedding. The Turtles also return in Turtles Forever, teaming up with their 1987 and later also Mirage counterparts to take down the Utrom Shredder, who was revived by the 1987 Shredder and Krang, once and for all.