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.
To write a Genos crossover fanfiction, think about the differences and similarities between Genos' world and the other universe. For example, if it's a crossover with a magical world, how will Genos' technology interact with magic? Develop your characters well, not just Genos but also the characters from the other universe. Make sure their interactions are believable. You can also add some humor or drama to make the story more engaging. Maybe Genos tries to learn magic but keeps failing in a comical way. This can add a light - hearted touch to the story.
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.
According to the information given, there were two novels about the dance in the Ultimate Class One, namely "The Four of the Ultimate Class One: The Rising Wind and the Clouds" and "The Ultimate Heavenly Gate". I hope you like my recommendation. Muah ~