Begin by brainstorming ideas for your OC. Decide on their appearance, personality, and goals. For the 'Red x OC' aspect, think about what kind of relationship they could have. Is it a friendship - turned - romance? Or a partnership based on mutual interests? Once you have these elements, start writing an engaging opening scene.
Well, you should do a lot of research on the world in which Red exists. If it's a game or a movie world, know the rules, the settings, etc. Then, when creating your OC, make sure they fit into that world in a believable way. For the relationship between Red and the OC, add some conflict and tension at the start to make the story more interesting. For example, they could start off as rivals and then gradually develop a deeper connection.