After struggling so much and overcoming so many battlefields, she had finally managed to become a fake couple with Zach. It might look like a fool's errand to the others, but Diana had invested herself fully in this fake relationship. She was in fact hoping that the fake relationship would eventually bloom into something real.
Just when Diana finally felt that she had a chance with Zach now that other bothersome girls were out of the way, another had come to take over her spot.
She felt that Zach already had a fake wife namely herself and didn't need another one as long as Diana was with him. She was determined to stand against such a decision even if she had to invest everything she had to achieve this end.
