Opposites Attract

No, he's not going to kill her. They're actually real close companions. He's not evil as he seems to be either.

Nox (left) and Zoa (right) are obviously opposites, the former being a serious and mature dark swordsman and the latter being just a carefree and innocent little girl, or at least that's how she is when she's not being rejected or ostracized by others. Also you should know that they both have dark powers; it's just that Nox has full mastery of it while Zoa constantly unleashes it at often inappropriate times when under stress or anxiety (not convincing at plain sight but that's her most significant trait).

Setting their differences aside, despite being generally very hard to gain Zoa's trust, they seem to have developed a family-like relationship quite quickly thanks to their one common trait of them having dark powers. Nox has made it an obligation to be Zoa's mentor, protector, and to an extent, her adoptive father, as well made it a mission to take Zoa out of the village that ostracized her all those years and somehow find a solution to resolve her lack of control with her powers. Nox is the first and only true friend of Zoa; the first and only person to actually show her love and compassion, something she's been desiring more than anything else throughout her ordeal.

I was originally going to add a background to make them be on a pathway in a dark forest but dropped the idea because then both their sides would be dark, defeating the theme of contrasting.