Leonardo

Leonardo Is the Oldest out of all the Five Turtles, he was considered the chosen one by Master Splinter to lead his brothers and protect them from any danger.

He is overprotective for Caravaggio, due to him being a long lost brother, who had been separated from the family for 19 years since the turtles were babies, and also considers that protecting his family would always remain his top priority.

2007 Monster Incident
Leonardo has been away for a long time, after being sent on a journey by Master Splinter to hone his skills in becoming a more efficient leader after the Shredder's defeat. April finds him in Central America, and once again, Leonardo's desire for perfection was shown, as even as his real training was over for a year, he didn't want to return because he feared he may still not be good enough. While he was hesitant to return to New York City, he does at the right time to take on the dangerous new force of evil.

His relationship with Raphael is strained due to Raphael feeling abandoned by Leo as well as feeling less appreciated by Splinter. Leonardo's vision of the world is perhaps wider than Raphael's; in the first movie prequel comic, Leo becomes angry with Raphael for trying to leave them in order to save a man from being mugged, because there are four heavily armed Triceratons in the sewers who could cause devastation to the city. He becomes further angered when Raphael deserts them mid-battle to help the old man. This conflict suggests that the two brothers operate on different levels of morality, though neither is necessarily wrong.

This also engages Leo in a contradiction when he stays in Central America, using violence to fight local lawlessness and effectively deserting his brothers because he believes, as Raphael believes, that others need him more. Such parallels suggest that the two brothers are experiencing the same dedication to justice but in different mentality, albeit in very different locales and using different tactics.

In the TMNT CGI movie, Raphael challenges Leonardo after arguing of their own individual sense of justice and the reasons for their actions. Leo also discovers that Raphael is the Nightwatcher and they engage in an emotional fight.

He is defeated when Raphael breaks his swords and pins him to the floor. Raphael almost kills Leo out of anger, then retreats in horror of his own actions due to shame and his brother's deep and confused stare. Leonardo is captured by the Stone Generals and Foot Clan, but is rescued by his family later before the final battle where Leonardo and Raphael finally resolve their differences. Raphael accepts Leo as their leader while Leonardo confesses to needing Raphael.

In the end he realized that he can do anything if he has his brothers by his side. Leonardo is voiced by James Arnold Taylor in this film.

Personality
Leonardo 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.

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.