This article is about the ability.
You may be looking for the Keychain.

Ultima Weapon (アルテマウェポン Arutema Wepon?) is an ability which appears in Kingdom Hearts 358/2 Days. It takes the form of a Ability Panel, and it increases the user's Strength when equipped. The amount varies depending on the equipped Gear.

Learning Ultima Weapon

Kingdom Hearts 358/2 Days