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 Organization XIII Mushrooms, the thirteen Mushroom Heartless scattered around the worlds in Kingdom Hearts II Final Mix+. They give you that Keyblade in the Hollow Bastion/Radiant Garden's canyon. This Keyblade, along with the Two Become One Keyblade, is a Kingdom Hearts II Final Mix exclusive.

Apperance
Winner's Proof displays exactly thirteen Mushrooms (four in the 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.

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.