Leonardo, often referred to as Leo, is the leader of the Teenage 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, 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 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 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 in Showdown, 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.
Fifteen years before Rise of the Turtles, Part 1, the benevolent Hamato Yoshi purchased four baby turtles. Yoshi came into contact with a rat and had to fight against a couple of Kraang. The aliens dropped their mutagen causing the mutation of Yoshi and the turtles. Yoshi raised the turtles as his own children and continues to train them in ninjutsu.
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. After The Invasion, Part 2, Leonardo's abilities were slightly lower, because of the injuries he obtained in his battle with the Foot Clan. However in Vision Quest, Leonardo managed to overcome his weaknesses and recover, regaining all of his lost strength.
- Leadership: Leonardo's bravery, determination, and willpower make him a great leader. In the beginning his only real flaws as a leader was due to his 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 a hero that results in him making rather poor taunts in battle. His greatest fear is to let the team down and fail as their leader. However, as he and his brother continued to fight against the Shredder and his forces, Leonardo become more focus and disciplined as a leader, even becoming willing to sacrifice his own life for the sake of others. He's also been able to make difficult choices whenever the situation came down to it. A note to make is that Splinter believed that Leo was destined to become the leader of his brothers ever since he was a small boy.
- Kenjutsu: Leonardo was trained by Splinter in the way of the sword. As his signature weapons are twin katana, he is highly skilled in the sword fighting style of the Niten Ichi-ryū,(二天一, "two heavens as one"), also known as Niten Ryu for short. He makes use of his swords for skilled offense and defense, allowing him slice through the metal of the Kraangs' robot bodies, fight against powerful and skilled members of the Foot Clan including Karai and Tiger Claw, and even hold his own against the Shredder himself.
- Tantōjutsu: Aside from his swordsmanship Leonardo is also skilled with a tantō. Whenever he finds himself without his either of his katana, Leo can take the tantō from the handle of one of them and use it for self-defense.
- Kyūjutsu: Leonardo also possesses aptitude in archery and uses a yumi and arrows to fight long-ranged.
- Shurikenjutsu: Leonardo can throw shiruken with remarkable accuracy.
- Strength: Although not a strong as Raphael, Leonardo his fairly balanced in terms of strength. His able to hold off and hurt opponents who are much larger and stronger than him. The greatest feat of his strength is when he faced off against Super Shredder in the final battle, able to knock him around with powerful blows.
- Speed: Leonardo is able to move at great speed, able to free run on rooftops without losing momentum. He also possesses great reaction speed, allowing him to dodge projectiles such as lasers the Kraang's blasters, and enemy attacks. He can also spin swing twin katana at incredible speed as well.
- Stealth: Being taught by Splinter, Leonardo,as well as his brothers, are able to be one with darkness and hide and move within the shadows without being noticed.
- The Healing Hands: In the events of The Deadly Venom, Splinter taught Leonardo ancient secret art known as the Healing Hand. With the technique, Leonardo is able to heal himself and others, and even remove Karai's venom from his body despite their being no cure for it.
During most of the series, Leonardo wields a pair of katana, one diato and shoto, which, like his brothers with their chosen weapons, he is highly skilled with. In Pulverizer Returns, Splinter says that the boys are getting too reliant 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 takes 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, Parts 1 and 2, when he was battling the Shredder's minions.
When fighting a mind-controlled Raphael in Clash of Mighty Mutantanimals, he shown a proficient use with a spear.
During his time in space, Leonardo used space katana blades and a laser blaster created by Fugitoid.
- 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: Rise of the Turtles, Part 1 and Rise of the Turtles, Part 2. They are very good friends, and April was the one Leonardo sought out when searching for advice regarding his crush on Karai. Leonardo's friendship with April is somewhat similar to a brother/sister relationship, when one looks after the other, like when April called for Leonardo to save her from the Creep in Within the Woods, and when Leonardo gives her hope in seeing her mother again in Buried Secrets.
- Casey Jones - Leonardo's second human friend. He met Casey in the eighth episode of the second season: The Good, The Bad and Casey Jones. Leonardo often calls Casey for help and Casey would come to help out. While they barely interact, they remain good friends.
- 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.
- 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 fascinated by her and has also come to consider her part of his family since she is the biological daughter of his adoptive father Hamato Yoshi.
- Michelangelo - Leonardo's youngest brother. He and Michelangelo don't fight, unless Michelangelo's slight dimwitted nature 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, Part 2, or Follow The Leader, when Leonardo actually goes back to save Michelangelo from danger.) They worked quite well together to acquire a worm to cure Karai. Overall, their brotherly-best-friend bond is unbreakable.
- 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 Showdown, Part 1, 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.
- 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.
- 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.
- Slash: Leonardo and Slash are allies, but not good friends. Leo views Slash with suspicion and distrust since the events of Slash and Destroy, while Slash is easily antagonized by Leo and prefers not to work with him. Nevertheless, Slash considers Leo a role model in his own efforts to be a good leader to the Mighty Mutanimals, and finally admits this to Leo at the end of Battle for New York, Part 2.
- Timothy - Leonardo never interacted with Timothy (the Pulverizer) as much, but he always gets annoyed whenever the Pulverizer tries taking too many risks that could get him injured or even killed (The Pulverizer Returns!).
- Shredder - Leonardo's arch-nemesis. Leonardo is most aware of Shredder's moves among 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.
- The Kraang - Leonardo's targeted enemy. 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 enemies. Relationship is unknown.
- Rahzar- One of Leonardo's enemies. It is a thought that he 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 Katakana is レオナルド (Reonarudo).
- Niten Ryu ("Two Heavens Style"), is described as Leonardo's weapons, but it is in fact the name of a fighting style for Japanese swords, 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).
- The existence of the ninjaken has been up for debate for many years.
- 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).
- According to the Concept Art of Leonardo's Katana, "Niten Ryu", one sword is longer than the other, and a Tanto blade is concealed within the hilt of the "Daito" (Long Sword), and a "Kogatana" (Mini Tanto) is concealed within the Saya's (Scabbard's) binding wrappings, all as secondary, auxiliary weapons to his Katanas.
- His mastery of Ninjutsu includes: Seishin teki Kyoyo, Taijustu, Qigong,Kenjustu, Shurikenjustu, Kayakujustu, Shinobi-iri, Sui-ren, Boryaku, Kyuudou, and Intonjutsu.
- 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) Rise of the Turtles, Part 1.
- His caller ID on April's phone is "ichiban turtle", which means "number one turtle".
- He is the oldest of his brothers; Raphael, Donatello and Michelangelo.
- 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 has a tanto blade hidden under his wrist wraps, as seen in TCRI.
- 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 The Invasion, Part 2.