Successfully complete the game under the indicated difficulty and ending. Then, load your cleared saved game file to access the costume at the options menu.

Bunny Girl (black): Any difficulty with good ending.
Bunny Girl (peach): Any difficulty with good ending.
Bunny Girl (red): Any difficulty with good ending.
Maid (black): Hard difficulty with good ending.
Maid (peach): Hard difficulty with good ending.
Maid (red): Hard difficulty with good ending.