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.
One way is to put Genos in a situation where he's outnumbered. As the enemies attack him from all sides, his defenses are slowly broken down. The writer can describe the attacks hitting him one by one, each causing more damage. They can also make the reader feel Genos' desperation as he realizes he might not be able to survive this time. And then, there's the aftermath. How does he deal with the pain and the damage? Does he become more determined or does he start to doubt himself? These are all aspects that can make a hurt scene for Genos impactful in a fanfiction.
Adventure is also a popular theme. They go on quests together, facing various challenges and villains. It adds an exciting element to the fanfic. Additionally, the theme of acceptance can be seen. Saitama accepts the female Genos for who she is, and she in turn accepts his quirks. This mutual acceptance forms the foundation of their relationship in many of these stories.