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DEF-Up is a positive status condition that has appeared in several Mario RPGs. It stands for "Defense-Up".
Mario & Luigi series
Mario & Luigi: Superstar Saga
Green Peppers boost defense in Mario & Luigi: Superstar Saga, meaning enemy attacks will do less damage to Mario and Luigi. The status is indicated by a gray box with an arrow pointing up next to the Mario brother's HP.
Mario & Luigi: Partners in Time
Green Peppers appear again in Mario & Luigi: Partners in Time, although they affect both fighters currently active. DEF-Up is indicated by an expanding green circle around the attacker.
Mario & Luigi: Bowser's Inside Story
Mario & Luigi: Dream Team
DEF-Up reappears in Mario & Luigi: Dream Team. Green streaks flowing upward and green flashing indicate the affected.
Paper Mario: The Thousand-Year Door
In Paper Mario: The Thousand-Year Door, several items and the Crystal Star power called Power Lift can raise defense. The DEF-Up status is indicated by a gray diamond with an arrow pointing up above Mario or his partner. Enemies can also receive this status effect.