Leonardo, often referred to as Leo, is the leader of the Mutant Ninja Turtles and the oldest of the brothers, who all share a home in the sewers 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 is the most serious, matured 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) and ssf5. 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 monkey and 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 their 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, 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 inand , where he almost sacrifices himself twice to save his brothers and the entire world. In the Season 2 Finale, and , Leo 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.
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.
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 , 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, Leo uses a pipe and holds and fights with it in a style similar to his Katanas. In and , he also used a Yumi bow and like the rest of his brothers wields shurikens.
- Raphael - Leonardo has a love-hate relationship with his brother Raphael. He and Raphael argue the most amongst all of the turtles, especially concerning who should lead the pack. However, they share deep respect for one another. In the episode , it is shown that Raphael is very protective about Leonardo and cares deeply for his brother, just like Leonardo does for him. It is seen that the two trust each other more than the others as seen how they often discuss things privately away from the others such as Leonardo's issue with Karai.
- Donatello - Leonardo's third youngest brother. He and Donatello rarely fight, but occasionally tease each other throughout the series. He usually gets anxious whenever Donatello completes an invention, one after the other, fearing that Donatello's technology might go wrong at one point. Donatello always outsmarts him, but never in leading purposes, annoying Leonardo in the process. Overall, they share a wonderful relationship as brothers and best friends.
- Michelangelo - Leonardo's youngest brother. He and Michelangelo don't fight, unless Michelangelo's slight dimwittedness irritates Leonardo, causing him to criticize Michelangelo's idiocy. Whenever Leonardo goes through a plan, he occasionally questions Michelangelo to see if he's really listening. Leonardo also feels very protective of Michelangelo, saving him from danger several times. (I.E. Showdown Pt. 2, or Follow The Leader, when Leonardo actually goes back to save Michelangelo from danger.) Overall, their brotherly-best-friend bond is unbreakable.
- Splinter- Leonardo's surrogate father and sensei. Leonardo always takes Splinter's advice seriously, even though at times, his stubbornness starts to annoy Splinter. Also, Leonardo gets confused whenever Splinter uses difficult phrases and quotes while giving away his wisdom, but he always takes it under consideration.
- April O'Neil - Leonardo's first human friend. He saves April from the Kraang as seen in the first two episodes of the first season: and . They are very good friends, and April was the one Leonardo sought out when searching for advice regarding his crush on Karai. They normally get along well, but they aren't seen interacting much.
- Casey Jones - Leonardo's second human friend. He met Casey in the eight episode of the second season: . Leonardo often calls Casey for help and Casey would come to help out. While they barely interact, they remain good friends.
- Leatherhead - Though they are allies, Leonardo and Leatherhead are almost never seen interacting, apart from fighting the Kraang. Leatherhead seems to interact more with Donatello and Michelangelo.
- Pulverizer - Leonardo never interacted with Pulverizer (Timothy) as much, but he always gets annoyed whenever the Pulverizer tries taking too many risks that could get him injured or even killed (see ).
- Karai - Leonardo was immediately fascinated by Karai from the moment they first met, and developed an infatuation with her. Leonardo eagerly wants to be able to trust Karai, even after it became apparent that she has repeatedly lied to him and his friends. Raphael disapproves of Leonardo's association with Karai, considering Karai a threat to their survival and Leonardo a liability for so frequently giving Karai the benefit of the doubt. Leonardo's ambiguous friendship with Karai was frayed when he tried to kill Shredder, and Karai revealed to him that Shredder was her father. Leonardo was the first family member Splinter confessed to that Karai was actually Hamato Miwa, his daughter. After Karai herself finally accepted the truth of her origins and tried to take revenge against Oroku Saki for his actions, she was captured and imprisoned by him, and Leonardo eagerly sought to rescue her. After Karai was captured again, Leonardo unsuccessfully tried to save her from falling into a vat of mutagen, which turned her into a snake mutant. Relationship-wise, Leonardo's attachment to Karai is two-fold: he is in love with her, and has also come to also consider her part of his family since she is the biological daughter of his adoptive father Hamato Yoshi.
- Shredder - Leonardo's arch nemesis. Leonardo is most aware of Shredder's moves amongst all his brothers, but still fails to fight him well. Shredder is much more experienced at ninjutsu and has more skills that Leonardo almost always fails to recognize, but Shredder only fails to kill Leonardo and his brothers, as they keep on escaping to the Sewers of Manhattan, NYC.
- Kraang - Leonardo's targeted archenemy. Relationship is unknown.
- Baxter Stockman - One of Leonardo's archenemies. Leonardo tends to forget Baxter's name (examples: "Dexter Backsman", "Daxter Boxman", "Blister Speckboy" etc.).
- Fishface - One of Leonardo's archenemies. Relationship is unknown.
- Rahzar - One of Leonardo's archenemies. It is a thought that is worried about Michelangelo battling him. When they find out that Michelangelo is in danger, he says that they have to save him. He was the one who cut the ropes off Michelangelo and then pats him on the head.
- Leonardo is named for the famous Renaissance master, Leonardo da Vinci.
- Leonardo's name is Italian, meaning "Strong as the Lion/Lion-Bold".
- His name in Kanji is レオナルド (Reonarudo).
- Given the fact that he was adopted by Hamato Yoshi/Splinter, Leonardo's full name would be "Hamato Leonardo".
- Niten Ryu is described as Leonardo's weapons, but it is in fact a fighting style, and not a weapon. A two-sword set-up is properly known as a daisho, (大小). Niten Ryu, instead, is short for Niten-ichi Ryu, (二天一流), a two-sword style developed by legendary samurai Miyamoto Musashi.
- Leonardo's swords, whilst being referred to as "Katana", are actually "Ninjatō" (忍者刀 ninjatō), (also known as ninjaken (忍者剣) or shinobigatana (忍刀).
- His mastery of Ninjutsu includes: Sheishienteki Kyoyo, Taijustu, Kenjustu, Shurikenjustu, Kayakujustu, Shinobi-iri, Sui-ren, Boryaku, Arhcery, and Intonjustu.
- Leonardo seems to be the most educated (out of the four brothers) about Japanese culture and etiquette:
- He understands Japanese when spoken to and is able to speak in this language to some extent.
- He knows, at least basics, of Japanese etiquette (i.e. he bows to master Splinter) .
- His caller ID on April's phone is "ichiban turtle", which means "number one turtle".
- He develops an obvious crush on Karai in the episode . He talks about her to April who is supportive; that is, until she finds out Karai is a part of the Foot Clan. He's the second Turtle to develop feelings for a 'human' girl, the first being Donatello (of course).
- Of all four turtles, Leonardo seems to be the most loyal and respectful to Splinter.
- He is the oldest of his brothers; Raphael, Donatello and Michelangelo.
- It's shown in that Leonardo can put things before his own brothers.
- He is the second tallest of his brothers, the first surprisingly being Donatello.
- Of all the turtles, he's the most mature:
- He was made leader by Splinter's choice.
- He sometimes acts like a parent to Michelangelo.
- He argues with Raphael, and eventually finds logical reasons to win the argument.
- Donatello and Michelangelo tend to listen to Leonardo instead of Raphael, because he thinks first before taking action.
- Unlike his brothers, Leonardo always takes Splinter's advice seriously.
- Leonardo is the first turtle to refer to Master Splinter as their father.
- Leonardo is the first of his brothers to learn that Karai is actually Miwa, Splinter's daughter.
- Leonardo has a tanto blade hidden under his wrist wraps, as seen in .
- Like all of his incarnations, except for the 1987 series (fear of snakes), Leonardo's fear is failure and losing his brothers.
- This version of Leonardo stores his swords on his back much like the original Mirage one, side by side instead of crossed.
- When Leonardo's voice actor changed between Season Two and Three, it was explained in-show as being the result of damage to his larynx during the final battle of .