A neat beginning to a story, but the "powers" the humans use is currently very vague, and I get the feeling it'd never be explained.
For an instance, how could Leo, a 12 years old, move the way she does? She leaps off of buildings and smashes 3 meters tall creatures like they are butter, while other people cannot. Why?
Are humans just more physically powerful in that world? A magical technique? Humans use magic? What is it, exactly, that makes them "strong"? And Leo, in particular?
The only clarification of 'power' we got was regarding Ragna's abilities, that he fused with silver. For an exposition, we sure got very little understanding of the most basic aspects of the settings. There's really no reason to keep the base limits of physical prowess of humans as a secret; it just makes me think that the author has no actual idea what "strong" means, so he didn't even bother to define it.
Also,
"I am the bone of my sword" much? kek.