Vopiscus Essair was his father. Deceased.
Ksenia Essair (nee Warwicke) is his mother.
Cyprian (formerly Kenleigh) is his elder half-brother.
Icarus and Cygnus are the other two in the same triplet set.
He has an uncle on his father's side somewhere in the Free Cities.
Most would consider Kassandros privileged. In some sense, he is, but in all the important ways, we use this term loosely. Perhaps, better than most, Kassandros realises and understands that Dalmasca's people are just as trapped in chains as her slaves are, in some ways, even more so. House Essair have been rather the name for many generations before Kassandros was born; the ilim that mostly raised him, and the House's head healer, Merenwen, says he was born cradled in sunlight, and Kassandros has never had a reason to doubt Merenwen.
As the eldest son of his generation, the eldest of his siblings, including his two twins, Icarus and Cygnus, Kassandros has always borne the weight of expectation, and a House as high as Essair has always had quite the list of them. As one of the major noble families in Dalmasca, these expectations are partly from the other nobility and the gentry, and partly by no one but themselves. Kassandros is an Essair, and male, and everyone knows those are grand miracles, given by the gods as a blessing upon those houses they are born in, or so the stories go. Kassandros always turned his nose up at such tales, but most of Dalmasca listens to them, and House Essair were no different.
By the tender age of only a year old, Kassandros had learned to walk, talk, and by two, used light sorcery the first time. His sorcery was somewhat rough, more like someone that was out of practice, rather than one whom had no knowledge to run off at all, and it took many more years before he came across something he wanted to do with magic, and couldn't. His visions began at a meager three years old, every once in a great while, and by the time he was five, they were nearly constant. If he spent long enough puzzling them out, as they were never direct, they told him almost everything that was going on at any given time; due to seers' overall rarity, none realised how strange that was, as a seer had an innate limit to how much information he or she could learn through visions before they died, and it'd seem Kassandros' limit was exceedingly high. He discovered, at four, that if he sat in the sunlight long enough, his eyes would start to glow, and the glow would last nearly all day, if he remained in direct sunlight for at least an hour. And the sheer amount of aura the boy was constantly storing, already immense, only continued to go up.
His mother, when he was six, decided she couldn't handle him anymore, and left House Essair. Kassandros hasn't seen Ksenia since, and some part of him kept holding onto the hope she'd come back, someday, but she never did. Merenwen took her place, raising him instead. It wasn't as if his father would do it, as he was too busy chasing man-tail and apparently deciding his son was screwing his life up, and needed to grow up before it was too late, and he doomed House Essair to failure.
Being the eldest, Kassandros was personally tutored in many varying educational subjects, in poise and grace, in etiquette and manner, eventually even being taught the foundations of combat (where, rather surprisingly, the slender young boy excelled, far more so than was expected, and Kassandros is rather the combatant when he chooses to be). It was well-known, firstborn sons eventually serve in the military, most often, when they come of age, and Kassandros would ultimately be no different. At first, Kassandros was adverse to this idea, but eventually he changed his mind, seemingly on a random whim.
In time, Kassandros started to gather a small following of others around his age, and eventually, that small cluster grew numbered enough to be rather the force. These became the start and foundation of the Cobra Syndicate, a criminal off-shoot of House Essair, the members of which pledged service to the House. After building their influence range over the course of four or five years, from the time he was eleven, Kassandros scattered them across Dalmasca, and seeks to slowly replace the law enforcement of the nation with his Cobras. Someday, all of Dalmasca's police force will be his. Nydema is already about two thirds overrun, with a large majority of the street patrols bearing the emblem of House Essair.
Amid this power shift, his father, in his intellectual ways, made the grave mistake of underestimating his son's capabilities and intelligence for the last time, claiming his son was doing nothing for the House and did not understand his place in it as the eldest son. Kassandros first attempted to ignore his blatant missing of how much Kassandros had done, both through the Cobras and through political ploy, to further the House's name and give him a headstart into the Warlord caste (at the time, House Essair were mercantile class, built primarily on wealth and goods transport), but something, eventually, snapped.
Some of the things he said made Kassandros start thinking about it a little harder. Ultimately, the conclusion he came to was this: if he was saying these things to Kassandros, whom he, ostensibly, favoured above his siblings, and there were two more that looked exactly like him, how long would it be, before he did more than just said weird things? How long until he pushed too far?
Kassandros was born first, to protect the ones that came after him. And damned if he wouldn't, even if it destroyed him to do it.
So, one day, Kassandros dropped an extremely toxic level dosage of a highly dangerous drug the Cobras frequently transported across nations, into his father's glass of wine, and sat back and let reality take care of the rest. Kassandros was fifteen, and he gave not a single fucking shit. It wasn't as if his father ever cared about him, always reminding him of the many expectations, and how easily he could misstep and screw everything up, and ranting and raving about how he needed to be more than just pretty.
Fuck you, Vopiscus Essair. Fuck you right into the underworld.
Since then, Kassandros has gone on military tour twice, once to a Dalmascan naval base in Jihon, and once to a rather heavily battered outpost in Macenia. Both times, he's made sure to make very lasting impressions on fellow soldiers serving, made sure to chat up the right ones, buy rounds of drinks for the right ones, and unfortunately, had a heck of a time fending off the wrong ones. He is, after all, slender and girly-looking, most think he's pretty easy to pin to a floor. Turns out, he is, if there's more than one of them.
He doesn't let it stop him. He doesn't have time to be upset about it. Or his father, or his mother, or anything else that goes not quite according to plan. He's got places to be, political ladders to climb, and an Imperator throne that needs his ass in it.
He's a seer. Yeah, he probably knows your character's name. Probably a lot more than that.
Rather powerful mage. Humes that are not already mages that spend a lot of time around him, eventually develop the ability to use magic through incidental aura-transfer. If you want to freak your hume character out, here's how.
A bit over two-thirds of the police force in Nydema, Dalmasca, are House Essair's Cobras. Are they corrupt? This depends on the definition of corrupt. If by corrupt, you mean does things that aren't quite legal, sure. If you mean brutal, horrible people that lash out randomly because they can, no. They're actually much better street patrols than the Dalmascan guard.
He is extremely kind to his slaves. Actually, he's probably only kind to his slaves, and maybe Livia and Lucius. Dalmascan nobles tend to get that faux-sweet facade, instead.
He has no understanding of his own emotions. He also doesn't display his emotions, and is nigh impossible to ruffle. He's getting better, just expect to get emotionless Ice Lord for a while, until, if ever, he trusts you.
Kass wears a sun necklace, rarely ever takes it off, made of gold, with an amber centrepiece, designed like a wheel (most probably assume it's a wheel). Those more familiar with Jihonese culture may recognise it easily as a sun, as Kirinyaga's solar imagery tends to look like a wheel, too.