Many of the available Ninjago T-shirts feature one of the main Ninjago characters.
Kai: Kai is the red Ninja of fire. He's the last Ninja to join Sensei Wu's team. Kai is the main hero and very popular with kids. Kai is a favorite with many kids including my older son who loves the color red. You'll find that quite a few of the Ninjago T-shirts feature Kai prominently.
Jay: Jay is the blue Ninja of lightning. Jay is the wise-cracking character. He's also very popular with kids. Because blue is popular and because of his fun character.
Cole: Cole is the black Ninja of earth. Cole is the team leader of the ninjas and has to try and get them all working together as a team.
Zane: Zane is the white Ninja of ice. He's a bit different. Um zaney, you might say! Zane likes to meditate and is full of eastern wisdom. But he often doesn't quite get the joke.
Sensei Wu: Sensei Wu is the teacher of the ninjas. He's the old guy with the white beard and the asian straw hat. He's my favorite!
If you're not sure who a child's favorite Ninjago ninja is then you might want to choose a T-shirt with a group picture of all the ninja together.