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.