They are just as supposed to know the victim, their character, and their history. There's no reason to suppose that a "close peer" jury would favor the defendant, it's just as likely to be the other way around.
On what is the claim that the jury is supposed to know any party based on?
Edit: To be clear, I agree with your premise that knowing the defendant does not necessarily bias a jury in favor of the defendant. I don't think there's any reason to assume the jury should have any knowledge of anyone, though. In fact if you do, you'll probably get removed from the jury.
