So what was Tristle did to Marcille? He may put some of his last 'desire' along with his life energy into her (because desire can't exist in dead body). This way he can keep 'living' in Marcille until someone doing something with the lion.
Long distance relationship is one thing, but what about Shinomiya's family? Thay don't talk about it at all like try to avoid that topic, which kinda sus here....