1.) Of course, everything in your character application must not conflict with the rules.
2.) No one is allowed to start out in the knights of rounds. You start as a soldier and work your way up in the ranks. That goes the same for the higher ranks of the Black Knights, some you can start as, others you have to work your way up.
3.) Only 6 members of the Britannian royal family will be currently allowed. Three Princes and three Princesse. Also, your character is not allowed to be a member of the royal family and renounce the throne, like Lelouche did.
4.) You may not start off with a geass, unless stated otherwise by an admin. Characers earn their geass by being active, good members and posting a lot
5.) Almost all characters in the show don't exist in this forum, so you can't mention anyone from the show in your history. (You can use a character as an example, like you say that your character wears the school uniforms like Lelouche, etc. but you can't have your character talk to Lelouche or mention him in any other way.
6.) Your character can't resemble a character from the anime too much. No cannon characters. (Us admins watch the anime, too. We'll know if your character looks like a character from the anime.)
7.) Your character can't be a character that gives out geass, or was a test subject for britannia. They also can't be a cyborg like Jeremiah.
8.) Your character has to have a logical background story. Like if you told a stranger your entire life's story, they'll most likely believe you. Nothing too ridiculous like that you're from another planet or dimension. (Parallel universes are purely fictional, by the way. And life on other planets is still a debatable subject to scientists and a lot of people. I'm only posting this because someone wanted a character from another planet, and when we wouldn't allow that, he wanted one from another dimension. Ridiculous, eh? So if it sounds unbelievible...it probably is.)