I'm... not sure you quite have the idea, to be honest. We are not agents. We are Boarders. We write stories about agents, who are fictional members of a fictional organization. That in mind, I'll try to answer your questions.

1. To write PPC stories, you need Official Permission, which is explained in the wiki article Permission. You should read it in full.

2. Good agents are interesting to read about, no matter what their species or how they dress. The ones I've enjoyed reading about the most are the ones who have especially distinct personalities. Just a few examples include Jay and Acacia (of course); Trojie and Pads; Suicide, Diocletian, and Ithalond; and Dafydd & friends. Characterization is properly the subject of an essay-length discussion, which I'm not prepared to get into here, but I hope that helps.

3. If you want to kill Mary-Sues, you join the DMS, whose agents are known as Assassins. That's pretty much it.

~Neshomeh