closed 
<3
[name_m]Fairchild[/name_m] is a prominent last name on the shadowhunters series. If you’re working on YA fiction, I’d steer clear of that. My favourite is actually LeMair. The slight alliteration of the L makes it roll of the tongue. From your favourites de Rossier is my favourite.
I agree with @rouverture that LeMair sounds the best. And while I don’t know much about Shadowhunters, I agree that you might not want to use a last name already strongly associated with a certain series. Plus, your other choices all sound decidedly [name_m]French[/name_m], which makes [name_m]Fairchild[/name_m] stick out. [name_f]El[/name_f]éonora LeMair has a great flow, though I like de Rossier and de Mercier a lot as well.
<3 Thanks guy