In children's dragon stories, you often find the dragon as a central character. Alongside, there are kids who are curious and adventurous. There could be a group of friends of the main child character. And sometimes, there are also some villainous characters other than the dragon, who try to use the dragon's power for bad things. The dragon keeper or trainer can also be a character, who has a special bond with the dragon.
The most common character is of course the dragon itself. It can be either good or bad. Then there's usually a young hero or heroine, like a brave little boy or girl. There might also be a wise old wizard or mentor who helps the child deal with the dragon.
I don't know for sure, but I would guess there are the children, and of course, the last dragon.
The main characters are George and the dragon. George is the hero who fights the dragon.
In 'How to Train Your Dragon', the main characters are Hiccup, a young Viking boy, and his dragon Toothless. Hiccup is small and not very typical of a Viking, but he has a big heart and a curious mind. Toothless is a Night Fury dragon, very intelligent and cute in his own way.
Foxes are also quite common. Foxes can be either sly or friendly depending on the story. Sometimes they play tricks on other animals, but in some stories, they are the heroes. Then there are the birds, such as sparrows and robins. They can bring news from different parts of the forest and their songs can add a cheerful atmosphere to the story.
Not having read the 'triangle stone dragon children's novel', it's difficult to name the exact main characters. However, given that it's a children's novel, we can expect children to be at the center of the story. They could be ordinary kids who stumble upon the mystery of the triangle stone and then cross paths with the dragon. There may be a group of children, each with their own personalities and skills. The dragon, which is a significant part of the title, is most likely a main character too. It could be a friendly dragon that helps the children or a misunderstood creature that the children need to befriend. There might also be some side characters like the children's parents or guardians who play a role in the background of the story.
I don't know for sure as I haven't read the novel. But I would expect there to be a child as one of the main characters, probably the one who discovers the portal and interacts with the puzzle dragons.
Perhaps it's a little boy who has a special pet. The boy and his pet could be the main characters, going through various experiences like helping their neighbors or solving a mystery in their neighborhood.
The main characters are Goldilocks, a little girl who enters the bears' house uninvited. And there are the three bears - Papa Bear, Mama Bear, and Baby Bear.
There are often brave knights. These are the ones who are usually at the forefront of the conquest, with their swords and shields ready to face the dragon.
The dragon itself is, of course, a common character. Whether it's in its original form at the start or in its reincarnated state.