Successfully complete the indicated task to unlock the corresponding character:

Gloria: Win all 75 storyline matches to get an invitation to the Love Tennis HQ. After you defeat Zeta (Gloria), you must defeat all Story mode characters in each stage (75 matches). Gloria will call your character and challenge you in a match at the Love Tennis Club HQ in the last room. Defeat Gloria to unlock her.
Helghast (North American and European versions only): After Colin joins in Story mode, go back to the stage room, and defeat him. He can also be used as an Umpire.
Suzuki: After Schneider joins the party in Story mode, go back to his stage. Talk to Suzuki, then defeat him.
Yakkun (Japanese version only): After Brad joins the party in Story mode, go back to his stage. Talk to Yakkun, then defeat him.
Zeta (Norma): After the first ending in Story mode, go to the volcano site. Defeat every player until the final door. Play as Emi to enter, then defeat Norma.

Play against the same CPU character, and give up three times consecutively to unlock the Easy difficulty.

Defeat the CPU on the Hard difficulty between one and ten times in Exhibition mode to unlock the God difficulty for that particular character in singles and doubles.

Defeat the CPU on the God difficulty in Exhibition mode to unlock the SP costumes and accessories for that particular character.

Defeat Colin at World Television Studio, then talk to Producer Bando again. Play and win a match against him to get the Panda costume as a gift from the Fans.