The hero is the central figure as the story is about his apprenticeship with the dragon. The dragon, being the mentor, is also a crucial character. There could be side characters such as villagers or other magical creatures that the hero and the dragon interact with. These side characters can add depth to the story, for example, a wise old wizard who gives the hero some advice from time to time, or a group of bandits that the hero has to face while on his journey with the dragon.
A theme could be the hero's journey of self - discovery. Learning from a dragon, which is often a symbol of mystery and great power in such novels, the hero may come to understand his own capabilities, his place in the world, and his destiny. There may also be a theme of loyalty, either the hero's loyalty to the dragon or vice versa, which can be tested in various situations in the plot.
Gohan and Piccolo are also very likely to be main characters. In the related movie, they had important roles, so in the light novel, they should be prominently featured as well. They both have unique powers and personalities that contribute a lot to the story. Gohan is a half - Saiyan with great potential, and Piccolo is a Namekian with his own set of skills like regeneration and powerful energy attacks.
The hero might go on a quest to find a magical artifact. Since the hero is apprenticed to a dragon, the dragon could guide the hero to ancient and secret places where such artifacts are hidden.
There's usually a central protagonist who is often on a journey to become a great hero. Maybe they have some kind of special power or potential that sets them apart.
The Shield Hero is of course one of the main characters. There may also be other heroes like the Sword Hero, Spear Hero, etc.
The main character is the forsaken hero, no doubt. There may be other characters like the ones who once trusted him but now doubt him. There could be a group of villains who are taking advantage of his forsaken state. And perhaps there are some ordinary people who start to see his true worth as the story unfolds, despite his being forsaken.
The main character is the Shield Hero. There are also other heroes like the Sword Hero, Spear Hero, and Bow Hero. And there are various supporting characters such as Raphtalia who becomes an important ally of the Shield Hero.
The main character is, of course, the forsaken hero himself. He is the central figure around which the whole story revolves.
Without having read the 'zero to hero light novel', it's impossible to accurately name the main characters. But if we think about common tropes in such stories, there could be a protagonist with an ordinary background, perhaps a student or a common worker. There might be an antagonist who stands in the way of the protagonist's rise to heroism. And there could be other supporting characters such as a group of friends who join the protagonist in their adventures and help them along the way to becoming a hero.