Stiffening gives Shikkui the ability to harden anything he touches.
By kneading objects together with both his hands, Shikkui can harden something like rubber balls to be as tough as concrete, to the point where they can break through solid rock if thrown with enough force.
Stiffening doesn't work on living beings. The hardened objects are also not acid-proof and can get stuck to viscous substances.
- This Quirk, in terms of function, is the inverse of Hardening, which only allows the user to harden their body.
- This Quirk, in terms of function, is the opposite of Softening, which consists of softening foreign objects instead of hardening them.