It's hard to say without reading it thoroughly, but perhaps there could be a detective - like character who is trying to uncover the secrets of Izaya and Namie. This character would add an element of mystery and suspense.
One main theme could be the complex relationships between characters. Izaya and Namie likely have a dynamic that involves mystery and intrigue. Another might be the exploration of their individual personalities, as they are probably distinct and interesting in their own ways.
Yui is quite an interesting secondary character. Her cheerful personality contrasts with Hachiman's cynicism and Yukino's coldness, and she often acts as a mediator between them.
One interesting secondary character could be their classmate who is always getting into comical situations. This character might add some humor to the story and also serve as a contrast to the more serious or deep - thinking Hirano and Kagiura. For example, when Hirano and Kagiura are having an intense conversation about their future plans, this classmate might barge in with a silly problem, like losing their lunch box.
Shino is often an interesting secondary character in Naruto fanfiction. His ability to control bugs gives a unique aspect to stories. Writers can explore his more introverted nature and how it affects his relationships with other characters. Another one is Rock Lee. His determination and unique fighting style can be the focus. In fanfiction, his story can be expanded on, like his training methods and how he overcomes his limitations.
Well, Caster can be considered an interesting secondary character. He has his own brand of magic and his actions often disrupt the flow of the Grail War in unexpected ways. Also, Lancer is quite captivating. His loyalty and the code of honor he follows, even in the chaotic Grail War, add an interesting dimension to the story.
Snape is an extremely interesting secondary character for fanfiction. His double - agent role and his complex feelings towards Lily Potter can be explored from different angles. There are fanfictions that focus on his early years at Hogwarts, his relationship with the Marauders, and how all of that shaped him into the man he became. Another character is Cho Chang. Her relationship with Harry can be re - examined and extended in fanfiction, and she can also be given more depth and backstory outside of just her romantic connection to Harry.
There could be some interesting mutants in the background. Maybe a young mutant who looks up to both Wolverine and Deadpool in different ways. This character might add an element of naivete and hope to the story.
Mairin can be really interesting. She has her own dreams and her relationship with Alain adds a lot to the story. She can be a source of motivation or a friendly rival for Alain.
One interesting secondary character could be Georgia's rival trainer. This character might have a very different approach to training Pokemon compared to Georgia. For example, they might focus more on the aesthetic beauty of their Pokemon's moves rather than pure power. Their interactions with Georgia could bring out different sides of her character.
Well, Pansy Parkinson can be an interesting secondary character. She might be jealous of Hermione's relationship with Draco and create some drama. Also, Ron Weasley can play an important role as Hermione's former close friend. His reactions to Hermione's newfound relationship with Draco in the context of the mafia fanfiction can be quite fascinating. And then there's Theodore Nott, who might be another figure in the mafia side of things, and his actions and decisions can affect Draco and Hermione's story.