Kizuna is probably the kind of hero Naofumi wanted to be.
Because of Naofumi's traumas and the attitude of the other heroes, hes let go of such expectations.
The great thing about Kizuna is that she shows that what Naofumi hoped for is indeed possible.
Kizuna is a great parallel to Naofumi, similar circumstances and yet such vastly different outcomes.
Kizuna opens up possibilities that Naofumi thought were impossible.