Play in Story mode or in Taisen Tetris to fullfill the requeriments to unlock the shadow characters that can't be selected in Taisen Tetris mode (for playing against the CPU or a second player).

Unlockable	How to unlock
Eriol, Ruby Moon and Spinel Sun	Finish Story mode (Easy or Normal difficulty)
Meilin	Finish Story Mode with 8 minutes remaining or enter the following as your name in the high-score board.
Sakura 2	Clear Stage 9 in Story mode (Easy or Normal difficulty)
Sakura 3	Clear Stage 17 in Story mode (Easy or Normal difficulty)
Touya	Defeat Sakura (CPU) with Shaoran in Taisen Tetris (Any difficulty will do)
Yukito	Defeat Ruby Moon (CPU) with Touya in Taisen Tetris (Any difficulty will do)