I'm not sure specifically, but there could be the Ninja Kid himself as the main character.
Most likely, the Ninja Kid is one of the main characters. There might also be a wise old ninja master who mentors the kid, teaching him the ways of the ninja. And perhaps there's an antagonist, like an evil warlord who wants to take over the ninja territory and is constantly in conflict with the Ninja Kid.
The main character is of course the Ninja Kid himself. He is often brave and resourceful.
There are two central characters in the 'the kid and the lion kid story'. One is simply a kid, who could be a human child or perhaps a young goat (as 'kid' can have different meanings). The other is the lion kid, which is a young lion. Their relationship and the events that occur between them form the core of the story.
The main characters in the ninja turtle story are the four turtles - Leonardo, Donatello, Raphael, and Michelangelo, and their sensei Splinter, as well as the villain Shredder.
Most likely, the main character is the boy ninja himself. There could also be his ninja master who teaches him the ways of the ninja.
Since it's Ninja Kids Story, the main characters could be a bunch of young and brave kids. There might be a leader among them, who is very determined and smart. Another character could be the funny one, always making jokes even in dangerous situations. And there could be a shy but very talented kid when it comes to using ninja tools.
Often, the main characters are the ninjas themselves. They could be young trainees starting their journey.
I'm not sure exactly, but likely there are some cute ninja girls as main characters.
Ninja Hattori is definitely one of the main characters. And of course, his master Kenichi should be there too.
The super ninja boy is the central figure. But there could be other characters like his best friend who is always by his side during his adventures. And perhaps an antagonist, an evil ninja or a powerful sorcerer who is trying to stop the super ninja boy from achieving his goals.
I don't know for sure, but it could be the ninja himself or herself as the main character. Maybe there are also some opponents like rival ninjas or samurais.