Eijiro Kirishima brings his Quirk beyond its limit to harden his body as much as possible. This transforms Eijiro into a form that is extremely rugged, giving him a monstrous appearance, but possesses great offensive and defensive power.
In this form, Eijiro can completely nullify most forms of attack. Sharp and blunt attacks that could normally break through Eijiro's standard hardened skin cannot pierce this armored mode. His attack power also receives a significant boost as his attacks become more fierce.
Red Riot Unbreakable, however, despite its augmented offensive and defensive properties, can be worn down with repeated hits, namely powerful ones. Kendo Rappa's rapid-fire punches effortlessly broke through this form. Eijiro can counteract this through strong will by continuously re-hardening after his defenses are broken.