Oz Vessalius is a great character to feature. His complex backstory, including his connection to the Abyss, gives a lot of room for exploration in a fanfic. Another good one is Alice. Her brash and often unpredictable nature can add a lot of interesting twists to a story. Also, Gilbert is a character full of loyalty and internal conflict, which can be used to create engaging storylines.
Definitely Xerxes Break. His mysterious past and his rather cynical view of the world make him a very interesting character for a fanfic. You can really dig into his motives and his relationship with other characters, like Sharon. Sharon herself is also a great choice. As a young and powerful member of Pandora, she has her own set of goals and relationships that can be explored in a fanfic. And let's not forget about Vincent Nightray. His dual nature and his complicated relationship with his brother Gilbert can be the basis for some really intense fanfic storylines.
Riku is also a popular character to feature. His complex past and his struggle between light and dark make for interesting storylines. You could write about his continued efforts to atone for his past mistakes and his role in protecting the different worlds.
Oz Vessalius is one of the main characters. He is a young nobleman with a mysterious past. Then there's Alice, a powerful being. And Gilbert, who is loyal to Oz.
Since I haven't read the Pandora Hearts Cacus Race Novel, I can't say for sure. However, in the Pandora Hearts series, there are characters like Alice who is quite significant. In this novel about the Cacus Race, there could be characters directly related to that race who play the main roles, perhaps a Cacus Race leader or a young Cacus who has some special destiny within the story.
Xander is a great character to feature. He's always full of energy and getting into fun adventures, which can make for an exciting story. Also, Emma, as she's often the voice of reason among the campers and can bring a sense of stability to the plot. And Lou, the counselor, is another good choice as she has to deal with all the camper's antics.
Oliver Queen is an obvious choice. He has such a complex backstory with his time on the island, his return to Starling City, and his evolution into the Green Arrow. His relationships with other characters like Diggle, Felicity, and Laurel also offer a lot of material for a fanfic.
Erina could also be great. Her refined palate and high standards add a layer of complexity. A fanfic could explore her journey of trying new and unorthodox cooking styles while still maintaining her elegance and grace in the kitchen.
The main characters are Break and Alice. Break is a complex character with a mysterious past and unique abilities in the Pandora Hearts universe. He is often seen as a bit of an enigma.
Syd is a character that can bring some humor to a fanfic. His interactions with Kat and his role in the story can be expanded. He could be involved in some comical misadventures while also being an important part of the group.
In my opinion, Bergy is an underrated character who could shine in a fanfic. He has that nerdy charm and his technical skills could be put to great use in a story. Maybe he discovers a glitch that is related to his area of expertise and he has to take the lead in fixing it. Miko and Five are also obvious choices. Miko's enthusiasm and Five's cool - headedness can create a great dynamic. They could be on a mission to stop a glitch that is spreading across all the arcades in town and along the way, they have to overcome their own personal differences as well as the challenges posed by the glitch.
I think Megumi would be a wonderful character to feature. She has a lot of growth potential in a fanfic. Starting from her insecurities and seeing her develop into a more confident cook. Also, Kuga can bring in an element of exotic and bold cooking styles. And don't forget about Akira, who has his own brand of precision cooking. Including these characters can add different flavors to the fanfic, both literally and figuratively.