In story mode, once Wendy joined the party, come back to her stage and talk to the bad guys near the athletic field. Beat them to unlock it.
Easy Mode
Play against the same computer player and give up three times in a row. There will be a message saying that easy mode has been unlock. You can choose to activate or deactivate it in the options menu.
Gloria
In story mode, after the second ending, go back to the volcano site and select to play as Suzuki before you enter the last room. Gloria will be inside.
God Difficulty Level
In exhibtion mode, beat the CPU at hard difficulty level a few times (the number is randomly set between 1 and 10). God Difficulty Level must be unlocked one by one for each character, in singles and doubles.
Helghast (US & EU Only)
In story mode, once Colin joined the party, go back to the stageroom and beat him. (He can also be used as an Umpire)
Lake Garden Court
In story mode, once Rosetta joined the party, come back to her stage and talk to the driver of the limousine. Beat him to unlock it.
Norma
In story mode, after the first ending, go to the volcano site and beat every player until the final door. Play as Emi to enter, then beat Norma.
SP Costumes and accessories
In exhibition mode, beat the CPU at God difficulty level. SP items must be unlocked one by one for each character.
Suzuki
In story mode, once Schneider joined the party, come back to his stage then talk to Suzuki. He will challenge you, so beat him to unlock him.
Theme Park Court
In story mode, once Tiger joined the party, come back to his stage and talk to the little girl near the entrance. Beat her to unlock it.
Yakkun
In story mode, once Brad joined the party, come back to his stage and talk to Yakkun. He will challenge you, so beat him to unlock him.