Complete the indicated task after unlock Poker Mode in Chapter 6 of Mikuru's storyline to unlock the corresponding costume.

Costume	How to unlock
Haruhi's Bikini	Defeat her Common Cloth version.
Haruhi's Common Cloth	Defeat her School Uniform version.
Haruhi's School Uniform	Defeat Mikuru's Nurse Dress version.
Itsuki's School Uniform	Defeat Nagato's Witch Dress version.
Itsuki's Swimsuit	Defeat his Thief Dress version.
Itsuki's Thief Dress	Defeat his School Uniform version.
Mikuru's Bikini	Defeat her Nurse Dress version.
Mikuru's Bunny Girl Dress	Defeat her School Uniform version.
Mikuru's Maid Dress	Defeat her Bunny Girl Dress version.
Mikuru's Nurse Dress	Defeat her Maid Dress version.
Nagato's Bikini	Defeat her Witch Dress version.
Nagato's School Uniform	Defeat Haruhi's Common Cloth version.
Nagato's Witch Dress	Defeat her School Uniform version.
Tsuruya's Bikini	Defeat her Maid Dress version.
Tsuruya's Maid Dress	Defeat her School Uniform version.
Tsuruya's School Uniform	Defeat Itsuki's Thief Dress version.