In order to unlock these weapons, enter tate mode and achieve 100 kills. For every time you do this, new gear will be unlocked and available for purchase:

New Mugen Weapon - 1st time
Jin's Firebird - 2nd time
New Worso Weapon - 3rd time
New Mugen Weapon - 4th time

Failing to reach 100 kills in tate mode will unlock new artwork.

Completion Unlocks

Finishing the game once with either Mugen or Jin will unlock the secret character Worso and deathmatch mode.