Marcus Cassimer is his father.
He does have a mother but I've forgotten her name, she is alive though.
Damian Cassimer is his adoptive younger brother. ... Lucius sort of half-raised him.
The eldest of House Cassimer, albeit he does not act much like it, Lucius was born, to much fanfare and hope. He likes to think most of his family were pleased when he was born, but quite frankly, it was probably very distantly, if at all. House Cassimer were never hurting for wealth, as Lucius' father before him had long snatched the 'most wealthy noble family in Dalmasca' title through real estate. Lucius' early years were easy enough, though he hardly ever saw his father, being raised exclusively by his mother, Lucius found he wanted his father's approval, and unfortunately, that led down a relatively difficult life path.
As soon as he was old enough, Lucius joined the Dalmascan military. Comprised of warlords and warlord-class nobility all vying for a potential spot among the justicars, and someday maybe a shot at becoming Imperator, the Dalmascan military is, to say the least, a land of cutthroat backstabbing and battles Lucius never did learn how to win. In truth, the only reason he's lasted so long in the military, and the only reason he's progressed so high in status, is due to how he treats the soldiers that are under his command. Lucius acts almost as if a father to them, nothing less. Among the warlord class, that's rare, to see in a commanding officer.
When Lucius was only in his early teens, a family friend of his father's came upon great misfortune. The family were almost entirely destroyed, and, with only one son surviving the house, as a favor to his friends, Lucius' father adopted the boy as his own. Lucius finally had a younger brother, and he was determined not to let his father's negligence ruin the boy, Damian, the same as it near ruined him. In all the ways Marcus was not a father to Lucius, Lucius was a father to Damian. Rather than attempting to shove him into a pathway that benefited their house, Lucius encouraged him to find what made him happy, what he wanted to do.
Of course, Marcus Cassimer ruined this conditioning at every possible turn, and sooner or later, it would've led to rather the gigantic fight. Rather than risk catching Damian in the middle of that, Lucius opted to take over management of a side estate in a smaller city a few miles north of Nydema the family held control over. For all Lucius is concerned, having learned compassion for fellow hume, not even just status-holding hume, but slaves too, through his falling madly in love with a brothel slave (shh, don't tell his father), and coming to be in possession of several dozen ilim slaves (to protect them from the perverse, you see), his father can die in a fire.
He'd never say that, of course, but by now, Lucius is tired of trying to be whatever Marcus seems to think he wants his son to be, and has long learned that, what that is, exactly, is quite bluntly a monster, and Lucius does not want to become a monster just for the sake of his father liking him again. They butt heads on many subjects.
If you must earn your father's love, that is not love, now is it?