In Kingdom Hearts II, Combo Boost is a support ability that increases the damage of a finishing move on the ground relative to the number of hits in the preceding combo. It costs 3 AP to equip, 5 AP in Kingdom Hearts II Final Mix.
In Kingdom Hearts 358/2 Days, Combo Boost is an equipment ability that increases the damage inflicted by each hit as the combo continues.
In Kingdom Hearts III, Combo Boost is a support ability that costs 5 AP to equip. It increases the damage of each hit in a ground combo by (number of hits) x 5%.
Learning Combo Boost
Kingdom Hearts II
Kingdom Hearts 358/2 Days
Weapons with Combo Boost
Rings with Combo Boost
Kingdom Hearts III