Unlock Reina:
At the title screen, press Up, Up, Down, Down, Left, Right, Left, Right, B, A.

Unlock bonus characters:
Complete story mode with Go to unlock Sieg Hart.
Complete story mode with Musica to unlock Read Admiral.
Complete story mode with Haru to unlock King.
Complete story mode with Elie to unlock Pasha.
Complete story mode with Shuda to unlock Gale.

Unlock Beast King:
Beat Story Mode on Hard using Haru without continuing. After round 7 King will change to a Beast, beat him to unlock The Beast King.

Unlock Outlaw
First beat story mode with everyone to unlock VS 2 enemies at once in Free mode. Then select Rear Admiral in Free Mode and set it to 1 on 2 with dificulty set to Hard. After you beat it, you will have unlocked Outlaw.

Alternate costumes
Successfully complete 1 On 3 mode on the hard difficulty setting with Elie, Haru, or Musica to unlock an alternate costume and portrait for that character.

1 On 2 Free mode
Successfully complete all storylines in story mode to unlock 1 On 2 Free mode.