This is a tough choice...
Muzy I think has more than earned the spot, being a great contributer here on SP, as well as a great encourager. His character provides quite a different element to the game and a unique playing option (after all, how many aboriginal, nature loving wild people are in the story and game?). So I think his should be in.
As to the other two:
Safir, he's a cool guy and has come off interested the few times he's come by lately (emphasis on "few times), but hasn't really been "into it" from what I've seen. His character is reasonably unique and does give possibilities for some different in-game strategies.
Sarah isn't an anime or manga lover (and thus not exactly all that "into" the story), but at the same time she's one of our few female members (and a notable member as well, and a cool person), and adds to the SP female roster in the game. Her character provides a unique element.
I think there is more in favor of Sarah's character, in the end. For instance, even though they're equally as undeveloped, her character is well connected with other members' in the manga (Kevin's and Ryan's come to mind, who are also in the game).
So, my other vote is going to Sarah.
It's tough. I like all three members so it's hard to have to help determine who gets the boot.
Maybe you could boot Procyon to make room for the third.
