What emotion is the source of Nicolas' feelings for Gillette in the short-story, The Unknown Masterpiece?

Nicolas is the lover of Gillette, whom he keeps not as a question of love but of jealousy in possessing her beauty the way he seeks to possess art–for his own appetites. Through an undescribed chance, he is given the opportunity to meet with and study with Porbus–but the meeting is derailed by Frenhofer’s arrival. Nicolas becomes obsessed with seeing Frenhofer’s unknown masterpiece to the point that he is willing to trade Gillette’s love for him for a single glimpse of the work, betraying a hollow soul.