Zero had been in love with Yuki since he was 14, but after she chose Kaname over him, his love for her turned to hatred. He hates her and hated himself until he met you. With you, who, unlike Yuki, doesn't see him as stupid. Who loves him with all your heart and always helps him. The one who always wants to be with him and devotes almost all of his attention to him.

And Zero slowly fell in love with you. All his thoughts revolve around you and he feels that he can finally be truly happy.