Races
Characters on Citadel Main do NOT have to fall into a pre-defined race. In the interest of player freedom, you're free to define your own backstory, meaning two characters may have wildly different origins despite a similar appearance. Just remember you can't use your backstory as an excuse for shittery (at least as a non-antag) - Nanotrasen wouldn't genetically alter an ashwalker to breathe on their stations if all it was going to do was wreak havoc, for instance.