Pro mode - Successfully complete the game to unlock the Pro difficulty.

Bonus costumes - Successfully complete the game on the Hard difficulty to unlock the Armored and Regeneration costumes.

Feel Good costume - In The Great Rift, find the circular room with ledges floating in it. Look down to see how it continues downward and upward. Jump to the exact middle top to find the Feel Good costume.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Easy Walk (10 points): Complete game at normal difficulty.
Everyday business (20 points): Complete game at Hard difficulty.
Tough fight (30 points): Complete game at Pro difficulty.
Oops, are you OK? (10 points): Kill your first monster.
Annoying fly (10 points): Kill a monster in the air using blades.
Take that! (10 points): Execute your first combo.
Can't touch me (40 points): Complete level without losing your health.
Can you EVER touch me? (60 points): Complete 6 levels without losing your health.
Flawless victory (50 points): Win a boss fight without losing your health.
Magic woman (40 points): Complete level using only magic.
Being good is easy (20 points): Complete game with a "good" ending at Normal difficulty.
Good girl (30 points): Complete game with a "good" ending at Hard difficulty.
It's hard to be a good girl (50 points): Complete game with a "good" ending at Pro difficulty.
Servant of the Dark (10 points): Learn all Dark spells.
Light adept (10 points): Learn all Light spells.
Finish him (50 points): Defeat The Dark One at The Dark Temple.
Counter impossible (30 points): Get IMPOSSIBLE hit counter.
Dark side (20 points): Execute Dark form for the first time.
Light side (20 points): Execute Light form for the first time.
Lock and load (50 points): Learn all shooting improvements.
Let's go shopping (30 points): Use every item at least once.
Nice silver thingie (20 points): Collect 3 silver artifact parts.
Nice gold thingie (20 points): Collect 3 gold artifact parts.
Nice ruby thingie (20 points): Collect 3 ruby artifact parts.
Shooting master (20 points): Get maximum Shooting upgrade level.
Melee master (20 points): Get maximum Melee upgrade level.
Airstrike master (20 points): Get maximum Airstrike upgrade level.
Master of all skills (50 points): Get maximum Shooting, Melee and Airstrike upgrade level.
Unbelievable metabolism (20 points): Upgrade health regeneration level to maximum.
I'm so special (20 points): Learn all Special Form upgrades.
Monster hunter (10 points): Kill 100 monsters.
Monster killer (20 points): Kill 1,000 monsters.
Famous monster eliminator (50 points): Kill 10,000 monsters.
Soul collector (10 points): Collect 5,000 souls.
Great soul collector (20 points): Collect 50,000 souls.
Epic soul collector (30 points): Collect 5,000,000 souls.
The best soul collector (50 points): Collect 50,000 000 souls.