Why does the bitzen in my elevator have a '?' on it?

The question mark characters are special characters. Each character "belongs" to a specific floor that matches some part of the story. If the character is delivered to the proper floor, you will unlock a cutscene for that floor.

A prime example is delivering the sticky buns Leia to the interrogation floor.

A question mark is in place of the bubble so that you do not know which floor they are going to, since you could just move their special floor to the floor they are going to unlock the scene. You have to get lucky with the character and floor they are going to.

Any question mark character can be delivered to any imperial floor, regardless of the number floor they are supposed to go to.

Unlocked special characters (Princess Leia, Boba Fett, etc.) are always question marks. From what I can tell, it means they'll never go to a residential floor with an open spot, since they can't move in. There are also regular bitizens with a question mark, but I don't know if the same applies to them.

I've noticed that "?" citizens double the tip when you drop them off at their requested level

For example, a regular citizen shows up to your elevator and requests level 10. You take them to level 10 and then you get a 10 coin tip.

If it was a citizen with a question mark who requested level 10 after the elevator moved, then you will get a 20 coin tip.

If you have any elevator upgrades, the multiplier (x2) of question mark citizens will apply after the elevator's {doubling, tripling, quadrupling, quintupling} is applied.

For example, lets say I have the x3 elevator upgrade. A regular citizen asks to go to level 10. When I drop him off at his requested level he will leave me with a 30 coin tip.

If it is a question mark citizen that requests to go to level 10 after I move the elevator, they will leave me with a 60 coin tip on arrival (10 x3(elevator multiplier) x2(question mark multiplier))

Bear in mind that this breakdown might not be accurate for the holiday-themed upgrade that I just got from Tiny Death Star, because it seems there is now a "figure-out-where-this-character-should-go" type mission for question mark citizens where you are rewarded with cutscenes and other goodies.

Edit: If you want to figure out whether you have the holiday-themed update or not: When you launch the game listen to the music. It will have a "holiday jingle" to it. Also your icon for the game will be covered in snow!

Bitizens with a ? can be delivered to any floor for 1 minute off whatever it's doing, a sales boost, and a double tip .

Additionally, if delivered to a related level, the game will play a scene and add it to your album. See Which characters unlock each scene? for where to deliver them.