By returning to completed dungeons, you can find the characters' secret weapons. Note: No secret weapon available for Chie. Note+: Main Character's secret weapon can be obtained early without fighting the extra boss by re-entering Void Ques before the boss appears.

Kanji's Secret Weapon - Defeat the extra boss in Steamy Bath House.
Kuma's Secret Weapon - Defeat the extra boss in Opera House.
Main Character's Secret Weapon - Defeat the extra boss in Void Quest.
Naoto's Secret Weapon - Defeat the extra boss in Secret Lab.
Yousuke's Secret Weapon - Found in the boss's den in Mysterious Shopping Street.
Yukiko's Secret Weapon - Defeat the extra boss in Yukiko's Castle.