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Background

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.