Genos is still kicking in the One Punch Man manga. The author has managed to keep him in the storyline without killing him off. There's always the possibility in the future, but for now, he's alive and fighting.
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.
I'm not sure which work you're referring to. Please provide more context information such as the title of the work, author, year, region, etc. so that I can better help you answer your questions.
You need to have a good story first. Then, focus on the character designs and make them unique and appealing. Also, pay attention to the frame composition and the use of colors.