Tidbits about Renard (and a little about Kyrenne) that Fiona knows:

Ren was the first to shapeshift; he and 'Renne were out playing in the woods one spring and he was pretending to be a fox...until, suddenly, he WAS one.  'Renne interrogated him as to how he did it and managed to change as well.  They ran back home - as foxes - to show their father, and caused chaos in the keep and manor when the guards tried to chase down the invading wild animals.  That summer, when Fee came to visit, the twins told her of their new ability.  They had less time to play that year though, because they were being tutored in shapeshifting by their uncle, Helven.  Since that time, fox hunting has been illegal on Lord Robert's personal grounds (though not on the estate as a whole); also, since the twins learned mouse form, mice are trapped and released within the manor, and there are no cats, on Lady Irina's orders.

Lord Robert liked to take the twins out hunting, tracking and fishing.  In the interest of starting Ren's martial training early, he had the master-at-arms train both the twins, as he know that Ren would never consent to learn without 'Renne at his side.  Fiona can immediately tell the difference between the twins - even when in mouse form, where they are virtually identical.  Fee also knows that only 'Renne can actually change what she looks like in human form; Ren has actively refused to learn that skill.

While 'Renne was off at Sianim (having shapechanged herself to look like a man) and Fee was at St. Catherine's, Renard was learning estate management and participating in war games with young nobles from other estates across Britain (including Scottish Britain, of course).  He was the best at the games, because he used his starling and mouse forms to spy on the other combatants.  He was also judged excellent at straight up combat, because of his emphasis on speed in combat.  He does well enough with a sword, but excels with twin daggers and bowmanship.

I've never given Ren a good physical description: he is a little taller than 'Renne, which still puts him at a height disadvantage compared to many other men his age.  He is lean in the torso but has well developed chest, arm and leg muscles.  He has the same color red hair as hers, and the same very green eyes and naturally fair skin, though he's usually tanned from being outdoors.  He wears a wolf tooth on a leather strap around his neck - they found it one summer, the three of them, and he won the subsequent contest to see who would keep it.

His hair brushes his shoulders normally, and has a bit of the same wave to it that his sister's would have if hers weren't so long.  He almost never pulls his hair back, and sometimes it falls in his face.  He enjoys hunting, reading, fishing, swimming, and has a weakness for sweets.  He, like his sister, prefers earth tones and greens and blues in his clothes; she wears black more often than he does though.  

His training as heir has revealed him to be an able manager, even if he doesn't enjoy it much.  Those who live and work on the estate are confident he'll be as good a lord as his father.  Because of this and his beauty - Ren is definitely prettier than most men - he had numerous young nobleladies suggested to him as a potential wife.  At larger society parties, they have flirted with him - some outrageously so.  One woman even tried to seduce him, to "trap" him into having to marry her.

He has never flirted back with any of these women, only politely moved on away from them.  The woman who tried to trap him found herself at arms' length of him in short order and then he strode down a hallway and disappeared on her (he went mouse form and hid until she was gone).  But it hasn't escaped anyone's attention that he always pays Fiona the most attention, that he smiles and dotes upon her.  So every other woman eventually wrote him off as Fiona's - some may have even congratulated her in private on such a fine catch.

Renard also dotes upon his younger siblings (though obviously in a different way from how he dotes on Fee).  He has four younger siblings: three brothers and a sister.  He's given sparring lessons to his brothers, and helped their tutors teach them.  He generally prefers peaceful solutions to problems, but can lose his temper when someone he loves is threatened.  Once when someone pulled his sister Jane's hair and made her cry, he threw the offending ruffian into the dirt.  And of course, he'd wanted to punch Roger Chillworth for breaking 'Renne's heart early on.  

His wrestling matches with 'Renne were always contests of equals, and were meant as harmless, goofy fun.  Any accidental cause of pain would bring the match to an immediate halt, no matter who was winning.  He dislikes alcohol, and has only been drunk once - it was such a miserable experience he refused to repeat it.  He drinks alcohol at social occasions where it is expected of him, and even then he nurses one drink the entire night.  Like 'Renne, he doesn't gamble per se: his only wagers are with his twin, and are usually more in the form of dares than things like betting on cards or dice.  

19 years old though, and he still pilfers sweets from the kitchens.