monday → in loco parentis
The easiest (and laziest) way to sum up actually a lot of Martel's drive throughout the Elenium is "mommy issues", which is accurate to a point! The family ties that are stressed - by the characters specifically and by the narrative itself - aren't blood ties, they're family created by shared experiences that incidentally falls into very particular nuclear family patterns. Martel and Sparhawk are both, themselves, only children; they still end up with this very specific dynamic where Martel has a (kind of hilarious) younger-brother-complex thing going on and Sparhawk reacts ... like you would expect an older brother to, both in the implications about what it was like when they were ~*as close as brothers*~ and now, now that Martel is the prodigal son who never came back.
So if Sparhawk is Martel's big brother to live up to (or fail to live up to), that makes Sephrenia his (their) mother, which makes Vanion everybody's dad. And that brings me to the actual point of this, instead of rambling for half an hour about Sparhawk (who is nice and all, but I can save that for another time).
The relationship that's stressed there is definitely Sephrenia more than Vanion, and I have explored the one more than the other, but I think there are moments in canon that are very telling about Martel's attitudes and emotions; he reacts to Vanion like even now he needs to prove something to him, albeit with defiance and spite. He says, in his last moments, that Sephrenia and Sparhawk were the only two people he ever truly loved - but as much as he'd probably like that to be true, it's not. They're probably the only two that, at that point, he's willing to admit (even to himself) any attachment to, and that had a lot to do with circumstance as much as anything else. Martel wants Sephrenia to love him and accept him, but he wants to impress Vanion; when he can't make him proud (and he did, once, clearly), he riles him up for a reaction instead, because he still needs a response. Nothing irritates Martel more than being ignored - in general, but particularly with people who matter to him.
His biological parents aren't discussed in canon, and I extrapolated a lot of headcanon to round out Martel's background to make him someone that I can conceivably play; in an early RP interaction, Candice asked Martel if it bothered his (blood relation) mother that he referred to Lady Sephrenia the same way, and he...completely failed to really understand the question, at the time, it took him by surprise. This typifies a lot of what I have for Martel and his parents; he respected them, he loved them, but it was slightly distant. They had their own world, and Martel had his, and sometimes the two overlapped. This probably contributed to why it was so easy for him to find intimate family somewhere else, but while their parenting style would be probably classified neglectful in a modern era, for the time and place it wasn't anything unusual and Martel doesn't consider his upbringing at all flawed. He had, like any spoiled nobleman's son, nursemaids who watched over him and tutors who oversaw his earliest education; Pandion novices go into it in childhood rather than when they're teenagers (that's considered extremely late to start), so he would've been very young and thus malleable when he went to Demos.
His parents (Romiar & Veleda) were wealthy Elene nobility; Romiar was from an established Pandion family, Veleda's family were relatively recent Arcian transplants, and Martel was born in Vardenais although his family relocated early in his childhood to be closer to Cimmura and Demos. The close ties to the knighthood and political situation in Elenia during Martel's youth would've put a lot of strain on their family: Veleda might have preferred if they'd washed their hands of it and gone to Arcium, or at least back to Vardenais, and a lot of Martel's courtly polish is owed to her pushing to maintain their social status in the face of a crumbling support network. Romiar, whose novitate came to an abrupt end when he took a nasty fall during a joust and shattered his hip, was more dedicated to both his country and the Pandions in particular, and admittedly while his wife was sort of like the socialite equivalent of a great white shark, he didn't actually give a damn about social politics and abused the fact that people would let him get away with a lot more ('the poor man is in pain, you know') to...get away with a lot more. He had a sharp tongue when he bothered, and most of the time he was more interested in a) his wife and b) the hunting dogs he bred on their property. He was, incidentally, where Martel got his signature hair colour from.
Martel is more inclined to discuss Sephrenia &/or Vanion than he is his birth parents; Romiar and Veleda had both already passed before he betrayed the Order, and he considers it a small mercy that they never had to witness his disgrace. He was the last of their family, and ... ruined the reputation pretty thoroughly, so this isn't entirely self-pity! It probably is better for them, in the end, not to have had to go through that. He appreciates the fact that they were proud of him when they died (unlike the parents to whom he was much closer - who couldn't be proud of him when he died), and to him it feels a little distasteful to discuss them now, like involving them in his life as it is would be disrespectful.
Between the two of them he really is quite a lot alike his parents - not in precisely the same ways, but there's a distinct family resemblance in more than just looks - but while Sephrenia also had a strong influence on him, this is more like the subtle ways that Vanion has molded the way Martel interacts with the world. Martel consciously and unconsciously models a lot of his work in Valdis currently on the way that Vanion led the Pandions - altered to suit the slightly different scope of his training school, but that's where the inspiration comes from, and as a matter of fact he (inadvertently) arranged his own tower study the same way as Vanion's. (Once he noticed he left it that way, like it'd be conceding that it mattered to him or had an impact on him if he changed it, which would obviously denote some kind of weakness.) Instead of keeping a personal journal, he has...a lot of unsent letters to his former Preceptor, none of which will ever see the light of day.
...man. LOOK, HE HAS MOMMY ISSUES. Okay, I'm done.