In the 'kabuto sad story', Kabuto could be anyone really. He could be a character from a small - town folktale, where he was known for his kindness but was wronged by the people in the town. Maybe he was framed for a crime he didn't commit, and as a result, he lost his reputation and had to live in isolation. This would be a very sad situation for him indeed.
First, consider the role the OC will play. If it's a partner for Genos, think about their combat abilities. For example, if Genos is mainly a physical fighter, the OC could be a support with long - range attacks or healing powers.
How about Alucard from Hellsing? He is an extremely powerful immortal. In a crossover with Genos, Alucard's centuries - old experiences and his complex relationship with humanity could play off well against Genos' more recent creation as a cyborg. They might start off as adversaries, but then find common ground in their fight against a greater evil. Genos could learn a lot about the nature of immortality from Alucard, who has seen so much over the years.
Female Genos could be developed with a more empathetic side. Since she is a cyborg, she could struggle with understanding human emotions more deeply. Maybe she starts to learn about love and friendship in a different way compared to male Genos. For example, she might be more sensitive to the feelings of other female characters around her.
To develop Fem Kabuto's character, give her a personal goal that is separate from Orochimaru's. In the fanfiction, she might strive to find a cure for a certain ninja - world disease using her medical knowledge. This not only shows her as more than just a sidekick to Orochimaru but also gives her a positive and independent identity. As she pursues this goal, she can grow and change, and her interactions with Naruto can be shaped by this new - found purpose.