There may be a mysterious guide figure in some 'blue lost girl fanfiction'. This character could be an old sage or a magical being that helps the lost girl on her journey. They might have special knowledge about the blue world she is in.
Often, the main character is the lost girl herself. She is usually portrayed as brave and curious, constantly seeking to understand her situation and find her place.
In many stories, there could be antagonists as well. Maybe a dark force that is trying to keep the lost girl trapped in the blue world. This could be a shadowy creature or a powerful sorcerer who has their own motives for not wanting her to escape or succeed in her quest.
Another common character type is the vampire - hunter. They might be lost in their own way, in a world where the lines between good and evil vampires are blurred. They could start off trying to kill all vampires but then have a change of heart after getting to know some of them better.
Often, you'll see Harry Potter himself as one of the main characters. Hermione and Ron might also make an appearance. From Lost Girl, Bo could be a main character. She has a strong personality and her Fae abilities can add a lot to the story when combined with the magic of Harry Potter. Dyson is another one from Lost Girl who could be involved. His loyalty and strength are traits that can play well in a fanfiction set in both universes.
It depends on the specific fanfiction. However, often there are characters that are associated with the themes of 'dark blue' and'moonlight'. Maybe a brooding hero type who is often seen in the dark blue of night or a mysterious figure that appears under the moonlight.
Often, the main character could be the person who owns or wears the blue jeans. They are central to the story as the jeans are usually closely associated with their experiences.
Typically, the main characters would be the girl who is the focus of the 'That's My Girl' concept and those around her who are important in her life. It could be her friends, family members, or a special someone like a love interest.
The Ender Dragon can also be a significant character in some fanfictions. It could be a menacing force that the Ender Girl has to face, or perhaps it has a more complex relationship with her, like being a protector in a way. And sometimes, there are other End - related mobs like the Shulker that play important roles in the story.
Since I haven't read the book, I can't say for sure. But they could be the lost boys and girls themselves, with each having their own distinct personalities and backstories.
There could be a mentor figure as well. This character could be an old wizard or a wise woman who has knowledge about the girl's blue eyes and the 'third' concept in the story. They could guide the girl and help her understand her powers and her destiny. Another possible main character could be a sidekick, like a young boy or girl who is loyal to the girl with blue eyes and helps her in her adventures.
I don't know for sure as I haven't read this novel. But in most Blue Bloods stories, there are often characters from the prominent blue - blooded families. So, it could be characters like Schuyler Van Alen or one of the Force family members if it follows the general Blue Bloods character line - up.
There could also be new characters introduced in the fanfiction. Maybe a female wizard who befriends Harry in his new form and helps him understand the female side of the wizarding world better. Or perhaps a villain who specifically targets Harry because of his unique transformation, seeing it as a weakness to be exploited.