In Kingdom Hearts II, Item Boost is a support ability that costs 2 AP to equip. Item Boosts on one character will not increase the effectiveness of items used by other characters. However, Item Boosts on one character will increase the effectiveness of items used on other characters. Item Boosts also do not affect items used from the menu; only items used in the field will be affected. Items boosts multiply the effectiveness of items by (0.5X+1). In other words, a potion (which restores 40% of the target's health) used by a character with two Item Boosts equipped will restore 80% to their target.
Learning Item Boost
Kingdom Hearts II
Kingdom Hearts II Final Mix
Kingdom Hearts Birth by Sleep
Kingdom Hearts 3D: Dream Drop Distance