Leonardo in the 2003 TV series is more like Leonardo from the Mirage comics than the 1987 TV series and first three films. He is more well-balanced and serious than in the 1987 TV series and first three films. Like the Mirage comics and the film penology, Leo and Raphael have disagreements constantly, particularly in the City at War trilogy.
Leonardo was a pet turtle bought by a young boy along with three others, Donatello, Raphael and Michelangelo. The boy was waiting at the crosswalk when a teenager noticed a blind old man crossing the street while a truck sped toward him. The teenager bumped into the boy with the turtles, causing him to drop the bowl in which he held them. The turtles fell into the sewer. Meanwhile, the teenager had successfully saved the old man's life. While the truck veered away from the scene, the doors flew open and a canister fell right into the sewer with the turtles, covering them with an odd substance. A rat by the name of Splinter was passing by and picked up the turtles, touching the odd chemical as well. When Splinter woke up one morning some point later, he found he and the turtles had grown taller, and slightly more intelligent.
Splinter thought it wise to train them in ninjutsu, just as his Master Yoshi had unknowingly done for him. Leonardo and his brothers were taken by Splinter to Japan when they were children to bury Hamato Yoshi's ashes with Tang Shen's remains, and alerted the Ancient One of his and Yoshi's condition. The turtles constantly noticed an odd demon visible to them, but invisible to the Ancient One and Splinter. Finally, when the Ancient One went to consult with the Ninja Tribunal, Splinter noticed an odd smell in the air, believing it to belong to the demon the turtles continued to see, and they secretly followed the Ancient One. The two ninja masters finally were attacked by the demon, but Leonardo managed to throw a stone at the amulet, causing it to shatter and reveal the demon. The Ancient One and Splinter quickly killed the demon. The Ninja Tribunal applauded the turtles' efforts, though eradicated all memories of the trip. The Ancient One gave the turtles bandanas, Leonardo's being blue. The Tribunal used their connection with nature to wrinkle space and open a portal to the turtles' current lair.
Later, Leonardo found Donatello pursuing a remote-control car that had gotten caught in the flow of the sewer water. Donatello's life was endangered by large amounts of sewer garbage, but Leo managed to free his brother. At some point, Splinter had discovered Leo was afraid of heights and trained him for weeks to conquer it. When it proved fruitless, Splinter faked a fall, which succeeded in forcing Leo to conquer his fear.
Even as a child, it was shown that Leonardo was extremely focused on his training, able to ignore Michelangelo's antics until the latter tried to fight him.
He is the leader of the group and the most "spiritual" of the four. He is kind of a grown-up, psychologically because of all of his seriousness.
He has a very close bond with Splinter, and has a strong sense of honor/Bushido and ethics, along with being a dedicated follower of the Bushido code. Episodes that deal with the Shredder and honor usually also focus on Leonardo, and he is often the Turtle who "saves the day" (in fact, Leonardo is the turtle who decapitates the Shredder early on in the series—of course, before they are aware that Shredder is in fact an evil Utrom by the name of Ch'rell who doesn't have a neck, let alone a distinct head).
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 in times of trouble. 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 the best in people, even potential enemies (i.e., Karai, Traximus and Quarry).
Though however cool and level-headed he may seem, Leonardo can easily be as irrational as Raphael at times. In "Same As It Never Was" Leonardo and Raphael both left the team, too angry with each other to be in close proximity. The start of season 4 shows a distinctly less cheerful and look-on-the-bright-side Leonardo, due to his feelings of failure, as did a short period in season one after the Shredder returns, in which he became depressed and angry over the loss of his swords.
In the fifth season,as he is a part of the eight acolytes under the Ninja Tribunal's training, Leonardo is the only one who doesn't receive a weapon from the Spirit Forge. The reason is unknown, but it's possible that while it saw him and Faraji as both worthy candidates, it sensed a greater force in Leo meaning he didn't need a weapon.
His otherworldly form is that of a dragon, a rare form, unheard of in someone his age. It could be a reason why the Spirit Forge didn't give him a weapon, and Kon, of the Tribunal, mentioned that he and the rest of the Tribunal were right about Leo, meaning they could have known he was given a great power that meant he didn't need a weapon. It is shown destroying evil guarding the second artifact with its ki alone. 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 by the dying Faraji Ngala. He returns Gunshin in episode 12 Enter the Dragons, Part 1 when Faraji returns to help battle the Tengu Shredder.
In the Fast Forward season, and the Back to the Sewer season, the damage that occurred to Leonardo's shell as stated above has somehow been repaired. Leo, Splinter, and his brothers are sent to the future by Cody Jones and Leo often calls Cody "little ninja". Leo still remains the most skilled and still dislikes Raph's reckless acts. However, he does not play as big a part in this season as in the previous five. He is still the level-headed eldest brother.
In one episode focusing on Leo, Darius Dun, Cody's uncle, made dangerous clones of the Turtles designed to kill them. In a fight, Leo's clone is hit with a laser by Donatello's clone. He is knocked unconscious and the Turtles take him in (under security precautions, of course). Leo gives his clone a chance to redeem himself and join Leo and his brothers, but the clone betrays their trust and runs away. However, it is later shown that Leo's words have had an effect on the clone, who now wants the other clones to stop living like animals and share, as Leo, Raph, Donnie, and Mikey have taught him.
Back to the Sewer
In the Back to the Sewer season, Leo and his brothers must find Master Splinter's data bits after Viral blasted him apart into Cyberspace. At the end of the series, Leo helped get Splinter back and goes to Casey and April's wedding. After that, Leo and his brothers battle the Cyber Foot and defeat the Cyber Shredder. Leo also sees Casey and April get married after all the Foot Ninja and the Cyber Shredder are defeated. Leo is still the most skilled in combat despite the fact that he did not kill the Cyber Shredder.
- Like his brothers, Leo is a practitioner of Chi Gong, as shown in Turtles in Space, Part 3: The Big House.
- In the season 3 finale episode, he was stabbed in the shoulder. In early season 4, when his bandages were removed, it was revealed that a portion of his shell had been hacked off too.