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.
The Raiden Shogun's sad story is complex. She was in charge of Inazuma and aimed for eternity. However, she faced many losses. Her past experiences, like the deaths of those dear to her, made her more determined yet also more lonely. She had to enforce rules that in the end created a divide between her and her people. She had to deal with the consequences of her actions when she finally realized that her idea of eternity might not be the best for Inazuma. All these elements combine to form a rather sad story for her.