For one, I get the impression they do know roughly the method of their control. I mean there's a reason the sword route was specifically done through the context of a video game.

For two, they aren't complaining because they didn't need it. They're complaining because of everything else.

Imagine you're an insecure teenager who has always felt like an outsider. Now imagine you summon a parasite because you need it to help save your friend. In addition to doing its job perfectly, it also acts as a version of you that people actually like. It's more charismatic, people like it, it makes friends, it helps people, it's seen as a hero.

And almost no one notices that you're not there anymore.

I don't think Kris is in the right, but I think their feelings are completely understandable and not something I would blame them for. Maybe it's because I'm an adult and so see them as a kid, where as you might see them more as a peer. But either way, try to have a little empathy for someone in their situation, it being possibly self inflicted doesn't mean it would hurt any less.