Here are the ways to get the ultimate weapon of each character in Kingdom Hearts. If you have any question or problems drop a message at the contact us page.

Sora's 'Ultima Weapon Keychain'
In order to synthesize the Ultima Weapon inn Traverse town, you first need to synthesize all 24 other items. After you've done that, you can synthesize the Ultima Weapon keychain. You need the following items:

5 Lucid Gems
5 Power Gems
5 Thunder Gems
3 Mystery Goos
3 Gales