Data Drain enemies and on rare occasions, you will get a secret weapon. Each secret rare weapon lowers one or more stat parameter by -10.

Secret Rare Twin Blade: Hyakkidorran (Data Drain the Parasite Dragon)
Secret Rare Blademaster: Jinsiran
The best weapon is acquired by Data Draining the Parasite Dragon on the airship. It does more damage than the Bom-Ba-Ye, is less accurate, but has more skills and magic power.