Please be aware that will obviously be SPOILERS for the entire series. And be warned that since readers/watchers have little familiarity with the series and scanlations are only in the first half of the manga, and readers who haven't read the manga in its entirety, it's best not to look at the spoilers in this character sheet.
Informed Attribute: Descriptions of his colouring are often at odds with his colour pictures and anime appearance. Early in the manga he is described as having black hair, 'the same grey eyes as his father' and dark skin (which is actually noted numerous times in the story), yet most of the artwork makes him brown-haired, brown-eyed and lightly tanned at most (see the picture on the main page for example). The anime and later artwork get the dark skin, but keep the brown hair and eyes.
Expository Hairstyle Change: A few. The first was when his previously straight-cut bangs grew into the more familiar asymmetrical style (which, inexplicably, happened halfway through a party), the second was when Auguste cut it short before sending him Off to Boarding School (but notably leaving the bangs intact), and the third was when he grew it out around volume 15.
Good Eyes, Evil Eyes: His eyes remain large and wide throughout his life, although during his childhood they became rather harsher, more angular and, bizarrely, square with his increasing jadedness and Jerkass tendencies.
Rape as Backstory: Oh, dear... long story short, poor Gilbert originally got raped when he was just nine years old - first by the paedophile Bonnard, and then by Auguste (who had some twisted motivations, to say the least) not long after that. It's a very big part of what left the kid so messed up.
Sex for Services: One of the reasons he hasn't flunked out of school is because he sleeps with classmates in exchange for papers... or just goes straight to the teachers.
Abusive Parents: Is one to Gilbert. And while his own adoptive parents were not quite actively abusive, they do appear to have wilfully overlooked the abuse he suffered at the hands of his brother.
Defrosting Ice Queen / Heel Face Turn: Becomes a lot more sympathetic as we start seeing his more vulnerable side, and he befriends Serge after a certain point. He eventually proves to be one of Serge and Gilbert's greatest allies, aiding their escape from Lacombrade when virtually the entire school turns against the pair.
The Dragon: Serves as Auguste's eyes and ears in the school, partly out of fear and partly out of self-interest.
Purple Eyes: Super-sparkly and completely purple in the OVA (seriously, the guy has no sclerae). Despite dialogue in the epilogue confirming that they're purple in the manga as well, he appears on a single manga cover with eyes that appear to be green.
Rape as Backstory: Specifically, he was raped by Auguste a few years before the story begins.
Single-Issue Psychology: Just about all his issues can be traced back to a single event in his past. (See above.)
Spell My Name with an "S": Is his first name Arion or Aryon, and should his surname be Rosemariné (accent or no), Rosmariné (ditto), Rosemarinay...?
Spell My Name with an "S": His first name has been spelt as Pascale (which is actually a girls' name, funnily enough) while his surname has been romanised as Biket.
Beautiful All Along, although not only was she not really that bad-looking to begin with, but Pascal insists that she was plenty beautiful enough already.
Parent with New Paramour: According to a flashback in the epilogue. Specifically, he found out that his mother and Rosemariné's father had a love affair going on. Awkward, to say the least...
Good Scars, Evil Scars: Has some noticeable scars on her face; she gained them following an unfortunate accident in which her hair caught fire when she was trying to save Serge from a punishment she thought he didn't deserve.
Temporary Blindness: Had this immediately following the incident where she got burned.
Unlucky Childhood Friend: She's been in love with Serge for a while; sadly for her, though, not only does he think of her as more like a little sister, but he also still feels guilty about her burn-scarring.
Expository Hairstyle Change: A couple or times. When he was young his hair was shoulder-length, as he got older he started wearing it shorter and following a time skip (during which Serge was born) he has grown it out and wears it in a ponytail.
Generation Xerox: He's essentially Serge with fairer skin and blond hair.
