"A buff that increases physical resistance while activated. Prevents Gon's Jajanken from being slowed by an opponent's attacks."
Fortify (堅 - Ken) is one of the Abilities available to use in Jump Force, and is one of Gon's abilities.
Overview
Upon activation, the user gains a defense boost for 25 seconds. This also gives Jajanken: Stance Super Armor; but only applies to Light Attacks. It also allows the user to perform two actions instead of just one from said stance.
Origins
Ken is a Nen technique which involves surrounding ones entire body, thereby protecting them from harm from others' Nen, especially Ko.
Fortify in action