You have to buy in Sakura's shop a full set of legendary hero costiume.
For unlock it you have to collect blueprints (collectibles).
Full set:
- Legacy Hood (5250, mission 40 "A Simple Mission", first mission in Akatsuchi Palace)
- Legacy Attire (3500, mission 20 " Secret of Daikizo Mine, first mission in Daikizo Mine)
- Legacy Trousers (3500, mission 12 "Rescue for Akatsuchi", first mission in Isolated Village)
- Orochi sword (3500, mission 4 "Aprroaching the Enemy", first mission in Highcastle)
(Edit by @TheCOMEBACKxKID, @Veuchh and @Leap Of Faith77 )