Fuse a persona during the fusion forecast 'Skill Change' to get the corresponding skill.

God Judgement:
Effects = 1/2 HP gone from enemies (Light).

Hell Judgement:
Effects = 1/2 HP gone from enemies (Dark).

When you have completed the game with the true ending start a New Game+ and a new fusion section will become available which will allow you be able to fuse Izanagi-no-Okami who is the Protagonist's ultimate persona.

The character that you are using gets their ultimate weapon on or after the 2nd playthrough when you find and defeat the Reaper of Death. The Reaper of Death can be found by opening treasure chests.

Each character has a secret weapon which can be found by returning to dungeons you have completed and completing the indicated task (there is no secret weapon available for Chie).

Unlock Main Character's Secret Weapon:
Defeat extra boss in Void Quest.

Unlock Kanji's Secret Weapon:
Defeat extra boss in Steamy Bath House.

Unlock Kuma's Secret Weapon:
Defeat extra boss in Opera House.

Unlock Naoto's Secret Weapon:
Defeat extra boss in Secret Lab.

Unlock Yukiko's Secret Weapon:
Defeat extra boss in Yukiko's Castle.

Unlock Yousuke's Secret Weapon:
Find in boss' den in Mysterious Shopping Street.

When you complete the main game once the 'New Game Plus' feature becomes availalbe where you start a new game with the weapons, items and stats from your completed game.