I'm surprised she doesn't resent her father at all.
She has felt lonely pretty much all her life due to his father's decision and even though her grandpa and grandpa most likely loved her a lot, it's not the same. If her parents had died, it would probably be different. They would be gone, so she wouldn't have a choice, but to seek love from her grandparents. But both of her parents are alive, and they are almost as unavailable as if they were dead...
Her wanting to have something special to her comes from that, imo. It's a way to try to overcome the loneliness she has felt for so long. Is love coming from that kind of place, real love? She cares for him, but the whale keychain probably made her think, "Do I like senpai as much as my dad likes whales? Or as much as grandma loves grandpa? Do I actually like him?"
Hopefully her parents get shit on by pigeons daily forever, even when they six feet under. Basically, the world saying, "Oh... we didn't shit on them today? Send the pigeons!" Dad's at fault for everything here, but her mom is also a work of art for completely disappearing from her life, even though she knew her ex-husband wouldn't pay attention to their child...