How to make an Ultrakill OC? [Art kinda unrelated]

I would love to make an Ultrakill OC, but I'm not completely sure on how to do it

If anyone would be so kind to give a brief explanation about machines and angels (or OC examples) I would love that!

Take in account I just beat act 2, so i'd rather not have spoilers (Earthmover is fine, I know about that)

Also I apologize if this is against the subreddit rules