Plus, as others like Zorbels and Karyuu have said, even when you're on good terms with the player, you'll tend to treat their new characters differently just because of who the player behind the character is. The easiest way to avoid this is simply to keep the identity of the player a secret. It's OOC information anyway.
Small side note since I likes to keep the facts straight.... Lady Z hasn't posted here yet, but I did refer to her as an exmple in my post.
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For me, I have the main character, but, I'm sorry to say, I get bored with playing 1 character. Be it vastly unfinshed game mechanics or not, months and months, and countless hours, even if its just a few here and there at a time, after awhile, things do start to get a bit stale. Rather than letting the RP atmosphere suffer, I then decide it's time to put this character down and try something else.
This game is supposed to be fun, not a second life. So after 3-4 months, as that's how long it normally takes for me to start to get bored and the value of my RP starts to degrade, I send my main off an exploration adventure. Then I crate a fresh character and start a different role. To keep it fresh, to have fun. I have tried both ways stated here. The first being telling people I play with, that this is just an alt of my main, and I'm just taking a break from my main, to play this character, and the other method. That being, keeping all ooc stuff out of it and just playing the role with out telling people this char is really just an alt of so and so.
From my personal experience, I like doing the latter of the two. Simply becuase, for the most generalist of players, they handle a new fresh character better in an IC way when they don't know that the player behind the character is really so and so. So I don't tell them. And to me, it seems to makes things more fun for me, and for the others interacting with my characters. Which is the point afterall, to have fun.
hope that makes sense, had to write a tad quickly. (I'm at work.)