Gustave! It's nice to have the wind magician's name finally revealed. He only appeared in a few chapters and I already love him. He also helped Lis grow up physically and emotionally which is quite over due.
So, love is every black magician's weakness. It makes them very weak and make their magic unstable.
I may be reaching here, but Gustave might be referring to Luna as the person he once loved. I mean, it makes sense to call him and Lis "accomplices" when it comes to their love for Luna.
There's also the bit where Gustave flying carpet was unstable as he hears Luna's words.
Either way, I hope he appears again soon.