Originally Posted by
fumei
She's his "sister", insofar as the two can actually be siblings. That's what she tells you at the ball. Later it turns out that she was taken in by Aurora as a kid and like Max said there's a certain implication that Percy ended up with her too in some form, but in general Lancelot also helped him out a lot as a kid, helped him train and all that stuff, and they just sort of developed a bond. In fact, that's the reason Percival does all that he does. He doesn't care about like saving britain or anything, he only wants to overthrow morgan so that lancelot won't have to be bound as a fairy knight anymore (since she seems miserable), though I don't think he knows that she maintains the form of a fairy knight so as to not just turn into the "monster" that is melusine or whatever.