Leonardo, nicknamed Leo, is a fictional character and one of the four main characters in Teenage Mutant Ninja Turtles comics and all related media. He is often depicted wearing a blue eye mask. His signature weapons are two katanas. Leonardo is the most serious, the most spiritual, the most disciplined, and the most in-line with Splinter's teachings and thoughts, and is the leader of the Turtles.
2012 animated series
Leonardo again leads the team in Nickelodeon's 3D computer-animated series. He was voiced by actor Jason Biggs up until "The Wrath of Tiger Claw", Dominic Catrambone for the remainder of the second season and Seth Green beginning in the third season. In this latest version, Leonardo seems to be more experienced and still perfecting his fighting skills and leadership abilities to make more solid decisions and gain more trust from his three younger brothers. He regularly watches an animated science fiction show called Space Heroes (a parody of Star Trek: The Animated Series) and uses it as a guide for his leadership skills, often attempting to quote from the show in an attempt to sound intimidating and heroic, even if most of his attempts fall flat due to him sounding overly cliche.  Leonardo immediately developed romantic feelings for Karai since their first encounter, even though she is his adoptive sister by Splinter; she was taken in by the Shredder after her mother's death and was tasked with destroying her birth family (including Leo) before changing sides upon discovering her true heritage as his adopted sister. His weapons here are purely dual katanas, which he uses in the Niten Ryu style of kenjutsu, making him an excellent swordsman. Despite the fact the other three turtles have added traits in this series, Leonardo is almost completely normal but now has blue eyes. Upon the sudden demise of his adopted father and master, he reluctantly steps up as sensei in addition to being leader, which puts even more pressure on him. He is visited, on occasion, by the spirit of Splinter who encourages him to lead his family and friends to stopping new evils.  In addition to his natural ninjitsu skills, he eventually developed the strong innate ability to heal via an enchanted mantra known as "the healing hands." By chanting the incantation and making the right hand seals, Leo is fully capable of revitalizing his inner strength and counteract even the most lethal of poisons and venom of "the healing hands." He managed to develop and utilize it to counteract the lethal venom of Karai, and attempted to use it on her to release her from the Shredder's control but failed. He then succeeded in saving Casey and Michelangelo from death. According to Splinter, he shows great gifts as a healer, and that being at the edge of his life had given him "a power that few martial artists can tap."
What is Leonardo's personality?
Leonardo seems to be more experienced and still perfecting his fighting skills and leadership abilities to make more solid decisions
Leonardo seems to be more experienced and still perfecting his fighting skills and leadership abilities to make more solid decisions