I don't have much of Fiona ironed out in my mind, but here's what I do have:

As mentioned: blonde haired, green eyed, pale with freckles, porcelain skin.  She's completely human, but doesn't care about Ren and 'Renne's shapeshifter blood; as mentioned, she's quite content petting Ren in his fox form.  She thinks he's adorable that way.  She's curvier than 'Renne (in all the right places, Ren would say).  She is also far, far more ladylike: she ALWAYS wears dresses, doesn't know any actual fighting skills (though she's gotten quite good at DODGING, being around Ren and 'Renne), is very proper.  She went to St. Catherine's convent school to improve her reading, writing and such.  She is, at first blush, the Perfect Medieval Woman.

While she is ladylike in dress and demeanor, she is just as straightforward in her talk as 'Renne is - or rather, she can be.  She knows when to demure and flatter and be coquettish.  Where as 'Renne has to fight and punch and kick and bite, Fiona can get her way through an artful word, through manipulative courtesy and the other, more usual feminine powers and wiles.  But those are tools that she uses; her normal mode of behavior is the plain speaking she used with Cederic.

She and 'Renne tell each other EVERYTHING.  For instance, though 'Renne claims to have also attended St. Catherine's, in reality she went to Sianim, a Scottish mercenary and spy training facility with her father's knowledge and blessing.  Fee backs up the story about St. Catherine's in public, smoothing over her best friend's obvious ignorance about the convent with her own knowledge.

Ren and 'Renne are 19; Fiona's 18 (18 and a half, really; Ren tries to pull the "I'm older, so we're doing what I say" thing sometimes, and she reminds him that 6 months isn't old enough to be smarter than she is).  When Fiona hit puberty and started noticing boys, she flirted around a bit - nothing serious.  She was enjoying getting positive attention from boys other than Renard - and really, his "positive attention" up to that point was the same schemes and plots and silliness he involved his sister in.

When Ren finally got through his awkward stage, he turned out pretty hot - red hair, green eyes, tanned and muscular from working outdoors.  She was a little worried he'd only think of her as another sister, given that they'd grown up together, but it was almost immediately apparent that that wasn't true.  So she dropped little hints and waited.  And of course, he was dense as a rock.  She has been biding her time, but was running out of patience when he finally proposed.

Fee, being a proper lady, would never think of having sex before marriage.  Of course, once married to him, THAT is a whole 'nother story. :3  Oh, and Ren has claimed to have spent time with willing women in the past.  Y'know, out with the guys, going drinking and whoring, that sort of thing.  Fee was worried about that at first, but 'Renne told her it was all a lie and he hasn't actually been with anyone.  Fee is gracious enough to play along for the sake of Ren's masculinity.

You don't fuck with Fee's friends or family.  She can't kick your ass, but she can find a million much sneakier ways to make you miserable.  To quote a movie, "The twinkle in the eye, and the arsenic in the soup."  (Well, maybe not arsenic - she doesn't want to KILL people.  But some sort of laxative root or something, quite definitely.)

I don't really know her personal likes and dislikes or anything else like that.  I know she's the only daughter of Lord Connor, but I don't know how many brothers she has.  I just basically pictured her as a Much More Ladylike 'Renne. Hell, she's probably responsible for teaching 'Renne how to act like a lady in the first place.