Assist is a new feature exclusive to Dissidia 012 Final Fantasy.
Assist works identical to the similarly named game mechanic found in Capcom crossover fighting games like Marvel Vs. Capcom. It allows the player to assign a second player and briefly call for their aid by launching a surprise BRV or HP attack. This can not only be used to perform new combos, but to interrupt enemy attacks, eliminating the need for the EX Guard altogether. The Assist is representing by a segmented yellow gauge beneath the Health Gauge which is filled up by landing successful hits or taking damage.
While Assist characters can be used to aid the user in attacking, it's also possible to use them as a shield against combos, known as Assist Change. When caught in an attack or combo, the user will switch places with the Assist to take damage, launching the user into the air freely to counterattack. When attacked, the Assist Gauge will lock up for a set period of time though this can be decreased or increased with Accessories.
It is also possible for an Assist character to be attacked directly. If hit even once, an Assist Break will occur in which not only does the opposing player's Assist gauge locks up, but the player will also gain the Bravery from the BRV Pool without needing to inflict Break on the actual opponent themselves.