You really still think their personalities are that similar? And both being practitioners of Chinese martial arts is a pretty flimsy reason for pairing two characters up.
And actually, if you want to get technical, Hsien-Ko gets her powers from the very fact that she is what she is, a magically-created creature and all. She's not limited to moonlit nights, and I'm pretty sure historical jiang-shi weren't either. And neither is Jon anymore, now that he's canonically able to change whenever he pleases.