For a canon character, just register with their name and your done, assuming they aren't taken. For Trophies that aren't playable in the game, post who you want here and I'll get you your bio page. Everyone can have 2 characters, and they can't both be in the same group.
Shadow Bug versions of characters are allowed, but they always are in the Villain group. Shadow versions learn all of the originals custom moves, but not the other way around, making them very dangerous. The downside is that if a shadow character, besides me, runs out of stock they get sent to the Great Maze and can't leave without a Boss' permission. And watch out, my job in the maze is to punish shadow characters for their failure, so get caught and you'll probably end up monkey food.
Besides the moves you begin with, you get to create your own vore moves, and by meeting certain conditions you can create custom special moves too. There's no limit to how many vore moves you make. So if you want to be a pred-based character you can load up on them, and if your more a prey type you can go without.
Besides you set character, everyone also gets an assist trophy. This can be any character that either is a canon assist, or just isn't in brawl, and hasn't been chosen as a main character. Assists can't fight outside of the Assist Arena, but can be called in once a match by their player. You can also request other assist players to interfere in your matches. Assist characters are invincible and generally perform one attack and leave. If they win enough in the Assist Arena, they can eventually become a full character, as well as still working as an assist.