The hikergirl is the central character, of course. But another main character could be the antagonist. It could be a greedy landowner in the Big Valley who wants to take over the land for his own profit. He sees the hikergirl as a threat to his plans because of her connection to the valley (blood runs thick) and her determination to protect it. Their clashes and the hikergirl's efforts to outwit him would form an important part of the story.
One possible plot idea could be that the hikergirl, who has a strong bond (blood runs thick) with her family or a group in the Big Valley setting. Maybe she embarks on a journey to discover a long - lost family secret. She could encounter various challenges along the way, like difficult terrains in the valley and unfriendly characters. But her determination, which is related to that strong bond she has, helps her overcome these obstacles.
One of the main characters could be the patriarch of the family, who is responsible for making major decisions about the ranch.
There are many possibilities for the main characters. It could be the valley's caretakers, if there are any, whose job is to maintain the balance in the valley. Or it could be a group of scientists who are studying the ecological system of the big valley. Another option could be a young couple who fall in love in the beautiful and mysterious big valley.
Often, the early settlers are main characters. They are the ones who started everything in the valley.
I don't know as I haven't read it. It could be original characters created by the fan fiction writer or it could be characters from the original work it's based on.
The main characters would include the blood witch, who brings her own set of powers and motives to the story. The Voltron team members are also key. Each of them has their own backstories and character arcs that will intersect with the blood witch's story. Additionally, if there are any leaders or authority figures in the Voltron universe in the fanfic, they could also be important main characters as they may have to make decisions regarding the blood witch.
Love and relationships also play a big part. There are stories about the romantic relationships between characters, as well as the friendships and rivalries that form in the small Western community. These relationships add depth to the characters and often drive the plot forward.
It could be a scientist from an opposing group who wants to capture Clarke and study the nanites in her blood. This scientist believes that the nanites hold the key to ultimate power and will stop at nothing to get his hands on them. He might send out teams to hunt Clarke down and she has to constantly be on the run, using her wits and any allies she can find to stay ahead of him.
Obviously, Percy would be the main character. But there could also be his friends who might be trying to find him after he runs away. For example, Annabeth could be a major character in the story as she would be worried about Percy and use her intelligence to track him down.
Since it's not clear what these stories are exactly, the characters could be anyone. Maybe local farmers, ranchers, or adventurers in the big valley.