Leonardo, often referred to as Leo, is the leader of the Mutant NinjaTurtles and the oldest of the brothers, who all share a home in thesewers of New York City. Leonardo belongs to a diverse family consisting of three other mutant turtles who are his brothers:Donatello, Michelangelo, and Raphael; and the ratlike Splinter, who serves as the Turtles' adoptive father and sensei. Due to his persevering dedication towards ninjutsu, (and partly because he asked for it), Splinter chose Leonardo to be the leader of the Turtles. He wields twin katana (or ninjato), indicated in the series as Niten Ryū.
Leonardo's bravery, determination, and willpower make him a great leader. He often tends to fight in a more traditional Japanese martial art style, and is good at countering attacks from opponents. Leonardo is stealthy like all his brothers, and like them he can hide in the shadows, sneak around without being detected, and use hand-to-hand combat. While not as agile as Michelangelo, strong as Raphael or smart as Donatello, Leonardo is more balanced. He is highly skilled in the sword fighting style of the Niten Ichi-ryū.
To date, his only real flaws as a leader are lack of experience (mostly due to his teenager status) - causing him to occasionally make impulsive decisions that might make the situation worse, as well as his brothers sometimes not obeying his orders, and a desire to be cool that results in him making rather poor taunts in battle. His greatest fear is to let the team down and fail as their leader.
As of The Invasion, Part 2, Leonardo's abilities are slightly lower, because of the injuries he obtained in his battle with the Foot Clan. However, as he recovers throughout Season 3, he is becoming stronger for the turtles' Return to New York♙
Leonardo is the most serious, mature and focused of the team. Being the designated leader of the group, Leonardo is very enthusiastic about his role. He is also a big science-fiction fan (or "nerd" as referred to by his brother Raphael). His favorite series is Space Heroes, while his favorite character from the show is Captain Ryan. Leonardo has a habit of memorizing Captain Ryan's best lines from the show and sometimes repeating them during battle. Leonardo is so caught up with Space Heroes that he relies on Ryan's character traits more than on his own intuition, which often causes his brothers (especially Raphael) to question his leadership.
For the most part, he tries to be the mature and responsible sibling, but there are occasional times where he can be as childish as his brothers, such as teasing Donatello for losing to a mutant monkeyand trying to be like an unrealistic TV character, although Leonardo often tries to comfort his brothers when they are scared or upset, mostly Michelangelo. His attempts to appear strong can also backfire as he makes rather cliché or ridiculous statements while trying to intimidate his enemies, such as introducing the team as the 'Mighty Mutants' or 'Turtles of Justice', while trying to appear heroic, or telling a scientist experimenting on a monkey that they'd had enough of him 'monkeying around'. Raphael often expresses his lack of patience for these quips.
He comes up with most of the team's plans and strategies, while he and his brothers are fighting bad guys or training in the Lair, but while these strategies have proven to be effective, his lack of practical experience has prompted his brothers - particularly Raphael - to become frustrated with his approaches, and they try to do things their own way instead. Sometimes, due to his oppressive burden as leader, Leonardo faces the fear of losing his brothers, though he is often calmed down by Splinter, who claims that the fear is held by almost all leaders. It is also shown that he kind of had a crush on Karai, but, of course, he cares for his brothers much more. In one episode, Leonardo told Karai that if she wanted to fight, she should fight him. He also said if she went after his brothers or Splinter, he'd go after her himself. This sentence indicates that he can be very courageous.
Leonardo is shown to be quite selfless when it counts. In the Season 1 Finale, this is especially shown inShowdown, Part 1 and Part 2, where he almost sacrifices himself twice to save his brothers and the entire world. In the Season 2 Finale, The Invasion, Part 1 and Part 2, Leonardo leads the Kraang away from his brothers and finds himself facing off against the entire Foot Clan alone. Knowing the dangers they'd face, he doesn't contact his brothers for help.
- In New Friend, Old Enemy When it shows a close up of Leo and Raph, right before they charge at the Foot soldiers, Leo has 4 swords. 2 in his hands and 2 in the holder on his back.
- In Within The Woods Leonardo's new voice - due to Seth Green replacing Dominic Catrambone as his voice actor - is explained in-show as being the result of damage to his larynx during the final battle against the Foot forces in The Invasion.
- In Within The Woods The creature, Creep, is river scum, not a leech like Bloodsucker. Creep was initially mutated by the mutagen-based medicine Donatello gave to Leonardo to help him heal. The medicine nauseated Leo and he vomited it onto the river bank and dropped the rest of the bottle. Bloodsucker's initial mutation was caused by drinking from Raph directly.
During most of the series, Leonardo wields a pair of twin Katana which, like his brothers with their chosen weapons, he is highly skilled with. In Pulverizer Returns, Splinter says that the boys are getting too reliable on their own weapons, which makes them swap. After this process, Leonardo is left with Donatello's bō staff. During a fight, the turtles become uncooperative with their new weapons and swap back. When Splinter finds out, he take away all weapons, stating that anything can be useful in battle. When Leonardo and his brothers are trying to save Timothy, Leonardo uses a pipe and holds and fights with it in a style similar to his Katanas. In Turtle Temper and The Gauntlet, he also used a Yumi bow and like the rest of his brothers wields shuriken.
The handles of his Katana blades also double as holsters for smaller tanto knives, as seen in "The Manhattan Project Part 2" and the season 2 finale, "The Invasion", when he was battling the Shredder's minions.