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Fast is a positive status condition in Paper Mario: The Thousand-Year Door. Some of Mario's enemies have the ability to power themselves up with this status condition, although Goombella can use Rally Wink to bestow it upon Mario as well (however, it will only last for one turn in this case). Indicated by a small "x2" above the enemy's head, it allows the enemy to attack twice in one turn. Macho Grubba uses this move to start his battle with Mario, and Beldam will occasionally use her magic on another partner in her second fight against Mario and his partners. When this status effect is used, it will say "More chances to attack are now available."
With the exception of Rally Wink or indirect causes, such as putting Beldam under confusion and theoretically having her give Mario or his partner the ailment instead of the intended target, the only other way to give Mario and his partner the status is hacking the game.
If a target has the Fast status effect and is inflicted with Slow, the two effects cancel each other out.