Ultimate boss

Ultimate Bosses are enemies that appear as completely optional bosses with powerful capabilities. What sets them apart from other optional bosses are the fact that they are the absolute, strongest enemies in the games they appear in, often requiring amazing levels of power and extensive strategies to defeat.

Ultimate Bosses are typically hidden, have certain conditions, and are always in the end game, if not during a second cycle. Some even require that several other optional bosses be defeated first in order to reach them.

Shin Megami Tensei I and II

 * Beelzebub (Neutral and Law route) any route in Shin Megami Tensei II

Majin Tensei II

 * Angra Mainyu (Neutral Route)

Shin Megami Tensei: Devil Summoner

 * Nanto Seikun and Hokuto Seikun (Must have finished the game)
 * Enoch, David, Leviathan and Squall (Must have finished the game, and defeated Nanto Seikun and Hokuto Seikun (PSP version only))

Shin Megami Tensei If...

 * Hazama's warped Mind Reiko Route only.

Devil Summoner: Soul Hackers

 * Devil Summoner protagonist (PlayStation and 3DS version only and only in the EX Dungeon after finishing the game)
 * Raidou Kuzunoha the XIV and Raido Kuzunoha the XIV (3DS version only and only in the EX Dungeon after finishing the game and after defeating the protagonist of Devil Summoner)
 * Soulless God Oumagatsu (3DS version only in the EX Dungeon. Must beat Raidou and Raido before facing him).

Persona 2: Eternal Punishment

 * Philemon (EX Dungeon only)

Shin Megami Tensei III: Nocturne

 * Lucifer (True Demon Ending's last boss)

Digital Devil Saga: Avatar Tuner

 * Hito-Shura (2nd cycle only, reach the end of the Karma Temple and return to the Anahata basement before fighting the final boss)

Digital Devil Saga: Avatar Tuner 2

 * Satan (Hard difficulty only)

Devil Summoner: Raidou Kuzunoha vs. The Soulless Army

 * Beelzebub (Reach the last floor of the training hall)

Persona 3 / FES / Portable

 * Elizabeth (2nd cycle only. Must accept her request and reach the top of Monad)
 * Theodore (P3P only, 2nd cycle only and with the Female Protagonist. Must accept his request and reach the top of Monad)
 * Margaret (P3P only, must have spared Ryoji Mochizuki on December 31 and completed all challenges in the Vision Quest.)

Devil Summoner 2: Raidou Kuzunoha vs. King Abaddon

 * Masakado (New Game +, neutral alignment, and 99% completion of Case Files)
 * Lucifer (New Game + and Chaos alignment last playthrough)

Persona 4 / Golden

 * Margaret (2nd cycle only, Empress Social Link must be maxed, and all optional bosses must have been defeated after rescuing someone)

Devil Survivor

 * Lucifer (New Game + only, on any non-Yuzu route)

Shin Megami Tensei: Strange Journey

 * Demiurge (New Game + only, Alilat must be defeated and the final Enemy Search must be acquired prior)

Devil Survivor 2

 * Alice (with Nebiros and Belial, the two having been defeated prior)

Persona 4 Arena

 * Elizabeth (last foe of Score Attack)