Conversation so far:
What is the animated series about for Leonardo of Teenage Mutant Ninja Turtles?
In the 1987 TV series' theme song lyrics, Leonardo of Teenage Mutant Ninja Turtles is said outright to be the leader of the TMNT, and there is little disputing this.
Did Leonardo of Teenage Mutant Ninja Turtles become more brave in the TV animated series?
Did Leonardo of Teenage Mutant Ninja Turtles fight against an enemy in the TV animated series?
Leonardo of Teenage Mutant Ninja Turtles is the most skilled of the Turtles, being the only one trained by two ninja masters, and capable of facing and defeating Karai.
Did Leonardo of Teenage Mutant Ninja Turtles defeat Karai?
Leonardo of Teenage Mutant Ninja Turtles returns to the city, reunites his family in a safe location, and confronts Karai. He defeats her.

Question: Did they ever try again to defeat her?
Self-contained rewrite: Did the Teenage Mutant Ninja Turtles ever try again to defeat Karai after her defeat by Leonardo in the fourth season of the 2003 animated series?