Leonardo

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.

Childhood
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 Leo conquering 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.

Personality
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 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 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, as did a short period in season 1 after the Shredder returns.

Trivia

 * 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.
 * He is right-handed.