Zara's actually one of the better-written LIs in SG. The look she gives MC when he tells her, "I believed in you", speaks volumes. I doubt I have to go further because I'm quite sure that the majority of her fans fell for her at that moment.
Nonetheless, I'll elaborate further. Everything leading up to that instance reveals her subconscious responses, shaped by her mother's influence. Essentially, the maltreatment she endured compelled her to behave in a particular manner, almost reflexively, around the clock. It wasn't until the arrival of the MC that she truly began contemplating her actions. Whether it's her reaching for his hand after the tennis match, subtly seeking his attention whenever they're together (which might also stem from a desire for validation due to her abusive past), or notably freezing when he expresses belief in her—these instances heightened people's interest in her, especially that final scene.
Her arrogance is a byproduct of her mother's conditioning. Either she excelled at something, or she faced physical, verbal, and mental abuse. The MC's presence in her life immediately challenges the version of herself created by her mother. Throughout her journey, she gradually evolves from that moulded persona to become her own unique individual. Beyond her toxic competitiveness, she possesses a delightful personality, reminiscent of Bella once you look beyond her rough exterior. Btw, if you failed to notice any of these nuances, my dear, it might be prudent to refine your observational skillsღ