Basic rules about characters.
1. Make them realistic. In other words, just don't make any bloody Mary Sues. Mary Sues are annoying and nobody loves them. Except maybe the people that made them. And even then their creators don't always love them (ie, parodies in which there are Mary Sues that are around to be the loathed villains). So please, no people with random blue hair and hot pink eyes that are drop-dead gorgeous (although blue hair and pink eyes are permitted for metamorphmaguses(?) in the Harry Potter rp, but not everyone is a you-know-what, so please!)
2. Please mind the naming. If a name is already taken, come up with a different one. Please. It just makes all of our lives easier.
And that should be it. I think that we are all intelligent human beings, so we should be fine.