There are a few restrictions to characters, but they're fairly easy to follow.
Because of the vast amount of designs in the world of My Hero Academia! Most creations are free reign design-wise, that being said...We encourage not to at least go overboard, we don't need anyone that looks like a digimon.
Don't make them too close to an already existing character, as nice as their design may be. You may be questioned when you send in your application. Don't make them look like some deity, don't bring fetish characters of any kind and don't bring in legitimate animals since animals wielding quirks is meant to be rare. That being said, anthropomorphic designs are okay, even if it's just a head.
No overly extreme sob stories. Sure, bad things happen in everyone's life. But this is not a contest of who has the best sob story. Do not write about specific rules for all quirk users, Do not