Guilt is a trait found on 6★ medals which have been fully synthesized after the player has cleared Mission 350 (In the Japanese version) or Mission 200 (In the English version). It acts as a second multiplier which further increases the power of a medal's Ability. There are three levels of guilt, depending on the rarity of the medal.
Activating Guilt on a medal requires another identical medal to be synthesized onto the fully synthesized medal, as if you were going to add a sixth orb to the medal. This will result in a random percentage being added to the medal next to the synthesis orbs, indicating the extra damage which will be added when the Ability is used. Guilt is particularly valuable on Partner Medals, which do not receive any Keyblade slot bonuses when used in combat. Synthesizing yet another identical medal will re-roll the guilt bonus percentage, but will not roll the percentage down from a previous percentage, if this is attempted, the percentage will remain what it was.
If the medal is common, it will likely have Low Guilt, which offers a 10-25% bonus to damage output; if it is a rare medal, it will likely have Medium Guilt with a 20-50% bonus; and if it is a premium medal, it will likely have High Guilt with either a 40-100% bonus or a 60-130% bonus.