Winner's Proof

The Winner's Proof (ウイナーズプルーフ) Keyblade is acquired after finding and fulfilling the tasks of the Mushroom XIII, the thirteen Mushroom Heartless scattered around the worlds in Kingdom Hearts II Final Mix. They give you that Keyblade in the Radiant Garden's canyon. This Keyblade, along with the Two Become One Keyblade, is a Kingdom Hearts II Final Mix exclusive.

Design
Winner's Proof displays exactly thirteen Mushrooms (four in the links and Keychain, two in the handle and the remaining seven on the blade (with one being the "sharp point" of the Keyblade). The Roman numeral XIII (13) is shown just above the hilt and signifies the 13 tasks you need to complete to acquire it. The head of the Keyblade forms a heart, bearing resemblance to the Ultima Weapon.

In Combat
It is the Keyblade with the highest bonus in Magic in Kingdom Hearts II Final Mix, surpassing Fatal Crest from the original Kingdom Hearts II.

Etymology
Winner's Proof's name comes from the fact that you have to pass thirteen tests, one for each Mushroom, to acquire it.