Hard to say but I think that MC is, and maybe the sister too. MC has superpowers - the parents do not. Also remember how Kajia freaked out when you put a bullet in the mercenary dude? She called down quickly but her original reaction was something along the lines of OMG, WTF did you do?!? And it was shortly after (after MC shed blood for the first time) that his powers started to appear. I suspect that the people you grew up with were your minders, not your parents.
There's definitely something up with him. He obviously has some sort of enhanced perception that allowed him to see the Damas (I think I got that right) and to realize something was wrong with the succubus' kiss and fight it off. There's also obviously something guiding him that may be tied into that. He does seem to learn things quite quickly too.
Pretty sure everything else (healing, reflexes and power in swordfights) is from Durendal.
Edit : Oh, and I stumbled into a Best Girl war.
Ok, I'll go ahead and vote for Nimue. I do like Katja and Fiona, though. And I really liked Finn.
Edit #2...ok wait, how the hell do you spell her name again? Is it KaTja or KaIja (with an i )?