You and Percy loathed each other for a variety of reasons.

He hated you for something ridiculous. you gave him a feeling that he had never experienced before. He gets a nervous feeling in his stomach whenever he sees you, and he detests that feeling.

As one of the few who witnessed the gods favoriting him, you detested him. He has only been at camp for a few days and he's already claimed? You were claimed but it took you MONTHS. That struck you as unjust. Also, he would constantly criticize you; throughout every engagement with him, there has never been a time when you two didn't argue.

You and your half-siblings were outside on a Saturday afternoon, enjoying the strawberry field. Just as you and your half-siblings were laughing and grinning while cracking jokes, you noticed Percy out of the corner of your eye. His eyes were on you. You chose to approach him as you thought it was impolite.

He stumbled as he said, “U-uh”