One of the central aspects to the Zero Time Dilemma game is the X-Door, the exit to the surface which only opens if six X-Passes are entered -- and each X-Pass is associated with one of the characters and is only revealed after they die.

Interestingly, in each different timeline of the game, the X-Passes are different, so each character ends up with several associated with them. And all of the passwords carry some special meaning for that character, typically special references to things that happen in this game or prior ones.

As such, this guide contains EXTREME SPOILERS. Looking at the passes themselves is reasonably safe (if you've just forgotten one when opening the door), but it's better to use the in-game LOG for that, both to ensure that you get the right password for the timeline you're in at the time (since they're different, and only the correct ones for that timeline will be accepted) and to avoid spoiling yourself on some of the story hints from other timelines.

There's another section afterwards that explains some of the password references, however. DO NOT READ THAT UNTIL YOU HAVE BEATEN THE GAME. Seriously, it contains ultimate spoilers that will ruin some of the game's best moments if you read them ahead of time.

This section is somewhat spoilerish. Read at your own risk. They're in no particular order.

This section is MASSIVELY MASSIVELY SPOILERISH. Do not read it until after you have finished the game.

• FIRE, FIGHT: He's a firefighter.
  
• SHIFT: He learns about how to SHIFT in this game.
  
• HERO, SAVE: As a firefighter, he's a hero. He also tries to be a hero during the game to save Akane & Junpei.
  
• DREAM: He has visions of several alternate timelines, which help reveal important secrets. Might also be a reference to his sister, who is trapped in a dreamlike state.

• CRASH, KEY: Akane is the leader of Crash Keys, which itself is a pun on her last name, Kurashiki.
  
• SIX, HEX: Her bracelet number in 999 was 6.
  
• JUNE: Her nickname in 999.

• FIVE: His bracelet number in 999.
  
• RING: He's carrying around one for Akane.
  
• HATE: (not sure about this one) He's much more cynical (and emo) than in 999 -- though not quite at VLR Tenmyouji levels yet.
  
• JUMP, DOLL: JUMPYDOLL, from 999. Also "Jumpy", Akane's nickname for him.
  
• QUARK: His adopted companion (grandson figure, though unrelated) in VLR.

WARNING: even if you have ignored all the previous warnings, know that this one contains ultimate spoilers. Absolutely do not read until you completely finish the game.

• EYE: Q is watching everything that happens in the shelter.
  
• AGE: Q is over a hundred years old.
  
• SELF: In several key scenes (and presumably throughout much of the game), Q is essentially acting as the player themselves, both as the viewpoint into the game world (using cameras throughout the shelter) and nudging characters to take actions that the player wants to try but they would not naturally do on their own.
  
• BRO: Q is actually Brother, the leader of Free the Soul. Also, he's Phi's actual brother.

• KILL: She is a serial killer.
  
• HEART, RIP: Her serial killer name is "Heart Ripper".
  
• VIRGIN: (not sure about this one) She hasn't been in love before.

• FOOL: He certainly acts like it.
  
• LOVE: He's in love with Mira.
  
• LAKE: He dumped his little brother's body in a lake after his father killed him.
  
• ICE: He works in an ice cream store. And the lake was ice-covered, since it was winter at the time.
  
• POOR: He comes from a poor background.

• MOON: Diana is the Roman goddess of the moon. Also references Luna from VLR, who was made in her image and named similarly.
  
• BLUE, BIRD: The music box that looks like a bluebird in a cage, also seen in VLR.
  
• MOM: Obviously, because she becomes a mother in ZTD.

• FUTURE, TIME: Her consciousness came back from the future. We later also learn that she was born in the future and physically travelled (in a certain sense) back in time, and then forward again, in a similar but opposite fashion to her later consciousness SHIFTs as explained in VLR.
  
• RED: She was the child in red. She also secretly has red hair.
  
• TWIN: She is a twin.
  
• DEAR: She is loved by many, especially Diana.

• FATE: He is trying to change the fate of humanity.
  
• DAD: He becomes a father in ZTD.
  
• OLD: His old consciousness has travelled back into his younger body.
  
• ARM: His older self had lost both his arms and replaced them with bionic parts.
  
• MILK: His last name is Klim, which is "milk" reversed.

Why the X? It's an eXit.

Another reason is that it's a hint that there are actually 10 people playing the game -- the 9 that you actually see on screen through most of the game plus Q, who is someone other than who you first think he is.

There's another punnish hint for this -- The Decision Game is not just called that because you're making decisions, but also because "deci" is the prefix for 10, similar to how the Nonary Game in prior installments required 9 people to play.

Let me know in the comments if I've somehow managed to miss (or just misplace) a password, or if you have a better explanation for one of the meanings.

Also please try to use spoiler tags if discussing one of the really big spoiler passwords, just in case.

Thanks to NumberC for the ICE password explanation and an alternate description for CRASH KEYS.
Thanks to W-Cactus for Phi's RED hair.
Thanks to Nineteen Letters Long for an alternate explanation about Q's BRO status.