Get sound test:
Buy the Sound Test for 5,000 points when you obtain the rank of S in the Normal Campaign.

Unlock Adder:
Complete any Campaign mode.

Unlock Lash:
Complete any Campaign mode.

Unlock Hawke:
Complete any Campaign mode.

Unlock Hachi:
Complete any Campaign on Hard mode.

Unlock Nell:
Obtain a rank of A in Campaign mode.

Unlock Jess:
Recruit Eagle or Drake.

Unlock Colin:
Recruit Olaf or Grit.

Unlock Sensei:
Recruit Sonja or Kanbei.

Unlock Grit:
Complete the Blue Moon campaign.

Unlock Olaf:
Complete the Blue Moon Campaign.

Unlock Drake:
Complete the Green Earth Campaign.

Unlock Eagle:
Complete the Green Earth campaign.

Unlock Sonja:
Complete the Yellow Comet Campaign.

Unlock Kanbei:
Complete the Yellow Comet Campaign.

Unlock Sturm:
Obtain a rank of A overall in Campaign mode.

Sensei:
Purchase Kanbei or Sonja at the store.

Grit:
Complete every one of the Blue Moon campaign levels to unlock Grit at the
store.

Eagle:
Complete every one of the Green Earth Campaign Levels to unlock Eagle at the
store.

Flak:
Purchase Flak in Battle Maps.

Hawke:
Complete all the Black Hole Campaign Levels to unlock Hawke at the store.

Hachi:
Complete all the hard mode Campaign Levels to unlock Hachi at the store.
Alternately buy everything in his store.

Jess:
Purchase Drake or Eagle at the store.

Kanbei:
Complete all the Yellow Comet Campaign Levels to unlock Kanbei at the store.

Lash:
Complete all the Black Hole campaign levels to unlock Lash at the store.