Obviously, Legion Haller is a main character. But there could also be other original characters created specifically for the fanfic. Maybe a sidekick who helps him in his adventures or an antagonist who tries to stop him.
A great fanfic idea could be to write about Legion Haller in an alternate universe. For example, what if he had different powers or was part of a different society? You could also focus on his relationships with other characters, like his friends or enemies, and how they develop over time.
David was this young lad who was taking care of his father's sheep. He was not from a military background but was chosen by God. In the story against Goliath, he showed remarkable bravery. He trusted in God's power and his own skills, like his ability with a slingshot, which ultimately led to his victory over the giant.
I'm not sure who David Fuller is in this story. There are so many people named David Fuller. He might be a character who had an important role in a particular event or a person with a unique personality in a family or social group. But without more context, it's difficult to be precise.
There's not enough given to know precisely who John David is. He could be a character in a family legend. For example, in a big family, there might be stories passed down about a great - uncle John David who had some kind of secret that affected the whole family's history. Or he could be a new character in a story being created, and the 'secret' part of the title is just to pique the reader's interest while his identity is slowly revealed throughout the story.