Honestly I think people misunderstand her, she isn’t normal the way other people are since she’s the hero. Amiyu is the one person on the planet who needs that drive to fight, she needs to lack that little bit of empathy and fear that everyone else has. That isn’t to say she doesn’t have any, just that it’s a lot less and it plays much less of a role in her mentality.
Amiyu is very capable, very strong, far stronger than most people who are a lot more experienced and older than her. And she got that strong by having that desire to fight, that urge within her which compels her to charge head first into danger to protect others, or to fight back against the evils of the world.
She doesn’t think anything of it because this is all normal to her, everyone else are the weird ones. And in some twist of fate Seika is probably the only person in the world who could understand her, but it’s that understanding which causes a bunch of misunderstandings on his part.
His desire to use her, exploit her compulsion to grow stronger and fight, treat her as a tool and form a bond which he can use to manipulate her. All of it lead to him slowly but surely growing attached in the way he tried not to be, he started protecting her, taking away any chances for growth she may have, preventing any battles she might partake in. She may be the hero, but even she is powerless to break out of that cage he’s putting her in, and it all started here.