In the Mirage Studios and 4Kids Entertainment 2003 animated TV series, Leonardo is voiced by Michael Sinterniklaas in the English version, Tetsuya Kakihara in the Japaneseversion, and Samuel Harjanne (seasons 1 and 2) and Markus Blom in the Finnish version. Leonardo is the leader of the group and the most "spiritual" of the four. He has a very close bond with Splinter, and has a strong sense of honor, ethics, and Bushidō. Leonardo's twin swords are slung across his back. Episodes that deal with the Shredder andhonor usually also focus on Leonardo, and he is often the Turtle who "saves the day". Leonardo is a more sensitive, self-doubting character than in previous incarnations.Raphael often quarrels with him and resents his leadership, sarcastically calling Leonardo "Fearless Leader", although the two are shown to be very close at times. Though Leonardo's relationships with his other brothers Michelangelo and Donatello are not as volatile, both have made comments alluding to the high standards the former has set, and his tendency to make them look bad. Despite this, his brothers view him as a pillar of strength and are at a loss when he is injured or absent. One of Leonardo's most prominent qualities is his determination to believe in the good and the best in people, even potential enemies; such as Karai.
At times, Leonardo is shown to be very hard on himself, as he feels that a lot is expected of him. As in the Mirage comics, Leonardo is ambushed and seriously injured by theFoot Clan and he feels he let his family and himself down. He has the same feelings after the final battle with the Shredder-his anger and self-doubt was caused by Karai, whom he believed was an honorable ally, but she was unable to go against her master's orders, eventually causing her to stab Leonardo (albeit unintentionally). Leonardo also feels extremely inadequate, as he believes that again, he let himself and his family down, this time by finding no other way to destroy the Shredder than to blow up the spaceship that both the Shredder and the Turtles were on; the Turtles and Splinter would have perished if they had not been rescued by Utroms. Eventually, Leonardo finds inner peace under the guidance of the Ancient One, who trained Splinter's sensei, Hamato Yoshi. From their final battle with the Shredder, Leonardo was the only Turtle to sustain truly lasting damage; part of his shell on his upper left shoulder had its edge shorn. Nevertheless, he is the most skilled of the Turtles, being the only one trained by two ninja masters, capable of facing and defeating Karai, the new Shredder, in a one-on-one fight, as well as defeating all three of his brothers at once in a sparring match.
Through much of the fourth season, Leonardo becomes increasingly stern with himself and adopts a greatly aggressive personality (which has been likened to Raph's previous impulsive and hotheaded ways on many occasions). Leonardo also shows considerably less reluctance in using violence to interrogate people, and devotes himself to even greater lengths of training. His brothers worry about him and Splinter feels he must move on. It comes to a head when Leonardo loses his temper and nearly causes Splinter serious injury during a training session. Splinter sends Leonardo to find Master Yoshi's own sensei, The Ancient One. Leonardo encounters a strange short man, as well as obstacles that echo his own anger. In the end, Leonardo admits that he was angry over failing his family while fighting the Shredder and that his only option was to self-destruct the ship to stop him. Leonardo comes to terms with his anger and begins training under the short man, who turns out to be the Ancient One. Leonardo only leaves when he learns that his family is in danger (a result of Karai's vengeance, which destroys the lair and presumably eliminates them). Leonardo returns to the city, reunites his family in a safe location, and battles Karai (giving her one last chance to leave the Turtles in peace).
In the fifth season, of the eight acolytes under the Tribunal's training, Leonardo is the only one who doesn't receive a weapon from the Spirit Forge. It is implied that his spirit is his weapon, and anything he holds is merely an extension. (This was hinted at in previous seasons.) His otherworldly form is that of a dragon, a rare form, unheard of in someone his age. It is shown destroying evil guarding the second artifact. This avatar is first shown in "More Worlds Than One". His brothers later exhibit dragon avatars as well. In the fifth episode "Beginning of the End", he is given the sword "Gunshin" (one of the Fangs of the Dragon that commands the "White Flame of the Dragon King") by the wounded Feragi, who believes the sword was meant to be Leo's. Leo returns Gunshin in episode 12 "Enter the Dragons" when Feragi returns to help battle the Shredder, because he believes the sword truly belongs to Feragi.
In the Fast Forward season, and the Back to the Sewers season, the damage that occurred to Leonardo's shell as stated above has somehow been repaired.
Leonardo is trained not just by Master Splinter but the Ancient One himself, Hamato Yoshi's trainer and adoptive father. From then on, Leonardo is far more experienced and skillful at even more complex ninjitsu moves than even Splinter, Raph, Mikey, and Donnie all at once. He cuts Shredder's head off in a one-on one-duel in Shredder's domain.
I have been raised to believe that words like truth and honor are more than just words, but one of the real difficulties in living a life of honor is that sometimes you might have to pay the ultimate price for it. - "The Ultimate Ninja"
You ever notice how there never seems to be enough time? Like when you get to play video games, or you're hanging with friends. Or when you're running like crazy to get away from a huge bomb that's about to explode! How the heck did I ever get myself into this? And... how the heck am I ever going to get out of it... alive? - "City at War", Part 1
There comes time in the life of the ninja when he or she must choose between the path that others would have them follow and the true path of their heart. Karai was raised by the Shredder, taught ninjutsu by the Shredder and she serves the Shredder still. But there's something about Karai that's different. She's not like the Shredder and because of that, Karai is going to have to make a choice between serving the Shredder and serving own sense of honor. I only hope she makes the right choice. - "Rogue in the House, Part 1"
Do you know what "irony" is? Irony is when you finally convince the Triceratons who invaded your planet to pack up and leave because the Fugitoid they're so desperately searching for is not on Earth, only to have the Fugitoid actually show up on Earth. Irony is when the very same energy scan that Don used to prove the Fugitoid wasn't on Earth is now used by the Triceratons to track down the Fugitoid. And finally, irony is when having succeeded in eluding your deadly alien pursuers, you find yourself surrounded by a bunch of heavily armed goons. And doesn't look like you're gonna make it out alive. - "Worlds Collide, Part II"
Fear. We face it everyday of our lives. Sometimes, fear can be a friend who fights by your side, making your reflexes quicker, senses sharper. Other times, fear is a liar, confusing you until you can't believe your own eyes. But the very worst fear: that's the fear that destroys all hope. And I'm afraid that my worst fear has just come true. - "The Darkness Within"
- Sword of Tengu
- Sword of Tengu Glove
The Creature shows the Turtles their worst fears, which are:
- Leonardo: Failure.
- Raphael: Himself - his anger.
- Donatello: The demise of his friends and loved ones.
- Michelangelo: Betrayal.