Costumes:
Hibana's 2nd costume - Beat the game on normal mode.
Hibana's 3rd costume - Beat the game on hard mode.
Hisui's 2nd costume - Collect all 88 Clan Coins
Characters:
Hisui - Beat the game on easy mode.
Hotsuma (JP version) - Beat normal mode while having a JP version
Shinobi game save in your memory card. The Shinobi save needs to
have finished the game at least once.
Joe Musashi - Beat 88 stages in the game altogether.
There's a counter at the bottom of the page in the rank screen
after you have completed a mission.