There are three endings:
Worst Ending: Defeat Dario Bossi (this is the second battle) without going into the mirror using the Paranoia soul.
Bad Ending: Defeat Aguni (go into the mirror in Dario's boss room), and then do not equip Mina's Talisman when going into the boss room in the Garden of Madness. This is a bad ending, but it unlocks Julius Mode. (This requires you defeat the Paranoia boss. To get to his room, you need to destroy three barriers by attacking them with three specific souls)
Best Ending: Defeat Aguni and equip Mina's Talisman before entering the boss room in the Garden of Madness. This results in two extra areas to play in and a couple extra bosses. This also unlocks Julius Mode and Sound Test.