Complete Monster: Seneca in the full game differs greatly from his Affably Evil version in the demo. A small yet malicious fox, Seneca aims to remake the remains of the world in his own image. He begins his streak of evil by physically and sexually assaulting the cat Charlotte, traumatizing her. When the heroes infiltrate his lair, he shows off his affinity for psychological torture, first by disguising himself as the Judge's dead brother Valerie to attack him, and then by trapping Ghost Batter in a small room while taunting him that, as a ghost, he can never escape the room even through death. He then presents the puppeteer with horribly mangled and suffering clones of the heroes that he created to test out his powers, before giving the puppeteer a demonstration of his soul-sucking machine that he uses to gain power. It is revealed here that, in the demo timeline, he tricked Zacharie into sacrificing himself to the machine to restore the world, a deal he had no intention of honoring, and that his prior kindness in said timeline was a façade. He then kills his scientist Louis for secretly helping out the puppeteer, before killing one of the clones for annoying him, and then expresses contempt for his creations, whom he beats whenever he feels angry. Finally, in the game's main ending, he turns a Brainwashed and Crazy Zacharie against the puppeteer in a last attempt to win. A sadistic dictator who outright denies feeling any remorse for his atrocities, Seneca proved to be far more wicked than even the Batter.
Also for beta players, the "voices of the past" in a video released by Claude advertising the upcoming update are Zacharie's screams of pain and terror, including a well-known line from the beta played backward.
Game-Breaking Bug / Good Bad Bugs: The Music Box is loaded with this, being an item that can send the player to the overworld regardless of location. From allowing Zacherie to leave The Room without having to fight Seneca to completely skipping getting the card for Zone 2, it can almost completely destroy the game. Which is possibly why the game removes it after entering Seneca's section.
Backtracking in general can break the game. Not only is Zachacloning (getting the storekeeper Zacherie to appear in Zone 0) possible to cause but "Not Safe" will always play in Zone 0 when the player enters it, the player can find The Judge if they go into the area with the red cube in Zone 0 (even if he's in their party), and the entire section in The Room will repeat itself, excluding the fight against Seneca, every time the player enters it.
Harsher in Hindsight: Seeing Shitai during the False battle can range between ridiculous or horrifying the first time around. After the events of 365, however, the battle takes on a more somber tinge, especially since you can immediately recruit Zackry after returning to the illusion hub.
Moral Event Horizon: Seneca seems to be a harmless goofball of a villain at first, but when he rapes Charlotte, it becomes clear that he is a cruel little bastard. He then goes on to commit several horrible acts, but it is his rape of Charlotte that pushes him into Not-So-Harmless Villain territory.