All characters :
At the title screen, press Square, Square, L1, L2, R1, R2, Square, Square to unlock all nineteen hidden generals.

All Shu generals :
At the title screen, press Square, R1, R1, R1, R1, R1, R2, . to unlock all Shu Generals.

Unlock all Wei Kingdom warriors :
At the title screen: square, square, square, square, R2, R2, R1, R1.

Edit mode :
Successfully complete musou mode with all characters except Dong Zhuo, Diao Chan, Lu Bu, Yuan Shao, and Zhang Jiao to unlock edit mode in the options menu. Alternately, press L1, L2, R2, R1, L1, L2, R2, R1 at the title screen.

BGM test option :
At the title screen, press R1, R1, R2, R2, L1, L1, L2, L2 to unlock the BGM test selection in the options menu.

Unlock Huang Zhong, Ma Chao, and Jiang Wei :
Successfully complete musou mode with any member of the Shu Kingdom.
[Submitted by:SnailB8ter]

Unlock Xiahou Yuan and Zhang Liao :
Successfully complete musou mode with any member of the Wei Kingdom.

Unlock Taishi Ci, Lu Meng, and Gan Ning :
Successfully complete musou mode with any member of the Wu Kingdom.

Unlock Diao Chan, Dong Zhuo, Yuan Chao, and Zhang Jiao in free mode :
Successfully complete musou mode with at least one member of each kingdom.

Unlock Side Selection in free mode :
Successfully complete musou mode with at least one member of each kingdom.