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When you beat the game once. you will start with the accessories
you had from the previous difficulty like helms, gauntlets,
earrings and hairstyles. Thats almost like a new game+

Purple Shinobi Shozoku that Ryu starts with only on your first playthrough.
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You will lose this suit at chapter 3 and
you can't get it back. Even on a new game
after you have beaten the game once.
If you want to stay as the purple shozoku
you will have to die three times in a row
and at the gameover screen
select to "abandon your ninja way".
NOTE: you have to do this before chapter 3.

This costume becomes available after chapter 3.
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You wont get another costume until you
beat the game at least once. On your new
game after beating the game once.
you will have the option to pick the battle suit from chapter 1.
This costume is called Legendary Black Falcon in the older titles.

This becomes available by beating the game on normal mode or hero mode.
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A throwback to the old ryu from the first three nes games!
NOTE: I still haven't beaten the first ninja gaiden on nes!

You unlock this costume by beating the game on normal mode or hero mode.
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This costume resembles the doppelganger fiends. Doppelganger fiends only spawn
on higher difficulties and mission mode.
NOTE: Increases your health but decreases your defense.

This costume is unlocked by beating the story on very hard mode.
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This is ryus outfit from the 3rd game.
NOTE: Increases attack and defense.

You unlock this when you get to
chapter 5 and get to play as Rachel.
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A very skimpy black leather suit.
There is no requirement for this costume.

You can unlock this costume by beating the game on normal mode or hero mode.
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A full gray outfit. Another day at the office
NOTE: Increases your health but decreases your defense.

This costume is unlocked by beating the game on hard mode.
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Gives Rachel a red leather suit with gold bazingas.
NOTE: Increases your attack but decreases your health.

Survival mode is also an Alternate way of unlocking things. All gold on Weapon Master Unlocks All Difficulties and Ryu's Costumes, Gold on Speed Master Unlocks all Ninja Trials, and Gold on Rachel Master Unlocks all of her Hairstyles.
~ Director Bison