Her father is responsible for all of her issues, thanks to seeing her adoption for several years as little more than a promotional tool for his business, then dumping ridiculous expectations and burdens on her (including the "no weakness" thing as well as the Very Stupid Thing we'll be getting to a few months further along the story timeline) as she entered primary school. He did, at a certain point, understand his error and grew to love her properly as her father, but not only had the damage been done, he has still at this point in the story, many years later, not actually explained any of this to her. Even while she's accepting the Very Stupid Thing even he doesn't actually want her to do any longer, he can't bring himself to show any weakness to his daughter so she'll relent until the entire cast conspires with Souta to leave him no other option.