I'm interested in the personal motivation aspect, that is, to save his (dead?) sister's child. Let me preface my comments to follow by saying that I do not want to stick my fingers into your pie and derail your train of thought. A character's motivations are stronger and strongest when they hit very close to the character's heart and soul. In the "Dororo" example, the main character is hunting demons to recover his own body parts.
My suggestion is to think about a couple of things. First, why did you decide to put his niece in jeopardy instead of his own little sister, his own child, etc. The further removed, the less gut-wrenching it is as a plot device. And if your heart is set on it being the niece, then think of how can you make it compelling. Such as, if he feels guilty for his sister's death because demons attacked the household and he failed to defend her so it's his fault that she's dead and he feels responsible for the niece and to make things right. Or you could establish that it's a female-centric culture and it's his duty to defend the maternal bloodline. In other words, he's honor bound.
At the end of the day, it's YOUR story and you should absolutely do whatever feels right to you.
I understand the removal aspect. I went with niece, because he was trained as a monk, and he had no wife or child of his own. His sister's husband died (war, or something similar) and the hero was raising the little niece like his own child, so even though he is 'uncle' he has been fulfilling the role of 'father'.
His sister is dead, killed by a hellion, which is what causes my monk to leave the order and go rogue, because they would not authorize his mission to kill the hellion, and gave it to someone else. It is that whole, 'this time, it is personal' scenario. He is honor-bound, but to his family first, which is another crime in the monk-hood, as he should love everyone equally, so he is banished anyway because he had a selfish love for another person, instead of a pure love for all. They are protectors of the peace and humanity, which is why they are not permitted to have families.
One of the other monks knew that he was seeing his sister and niece (sneaking out at night) but he allowed it, because he had hoped that this would show him the value of love, but it turned against the hero, who now, has to try and free the soul of his niece, because his sister's dying words were to 'take care of her', because she did not know that they demon had taken her into the soul trap.
Anyway, that was my idea, though, I question how this will all play out. I have begun the take yesterday, and continued this morning, so I will see how this all plays out in 80K words (I hope)
