Okay, I've only seen Buffy up to and through the third season.

I know that in the fifth season she acquires a little sister named Dawn, who has apparently been there all along, though we've never seen her. This is because she is "The Key" and only just arrived, but everyone's memories were tampered with so that she seemingly has existed since she was born.

Just how many memories have been tampered with?

Does Angel remember Dawn? (Since she didn't arrive until after he moved to LA) Cordie? Wesley?

Does Hank pay child support for her?

Are the viewers ever filled in with what Dawn was "doing" during the previous years, like when Angel lost his soul, or when Buffy ran away after sending Angel to Hell, or the big battle with the Mayor?

I thought Dawn was new, meaning the only memories tampered with were Dawn's. She has memories that none of the others have but those memories aren't real. In "real life" she never was really there but she believes that she was because she has the false memories. I could be wrong but that's the way I understood it.

Dawn isn’t real?? WTF!

I’m gonna try this, even though I usually get something twisted and will have to be corrected later.

Dawn is Buffy’s sister. All the memories are in place (or “tampered with,” as you say) to support that. kilcher, everybody else has “false memories,” too.

How many memories? Does it matter? If you suddenly find yourself with a whineybutt little sister that you didn’t have a few minutes ago, will I know the difference?

Does Angel remember Dawn? Well, of course he does, and fondly. He has memories, when he was Angeles, of raping the crap outta her.

Does Hank pay child support? $235/month.

Do we ever get to see Dawn’s past? Not that I recall. Make something up. It worked for me.

Hope this helps.

Everyone on the show has memories of Dawn

It's never mentioned if the cast of Angel knows her from what I can remember. Angel only pops up on Buffy once after Dawn is introduced and his scene is with Buffy only.

Not sure about Hank

Random memories are talked about but nothing about what Dawn was doing during any of the big events.

Also remember when they were going to do the cartoon, Dawn was going to be in it. And that is set when Buffy was still in Highschool. I don't remember if Angel mentioned Dawn in the episode Forever, I would have to rewatch it, but it wouldn't surprise me.

I may be wrong about this, but I remember getting the impression from an episode last season (the one where she's in the mental hospital maybe?) that her dad Hank was also dead.

He's not dead as far as I can remember. He just went from a loving, caring dad to an absent father who didn't even come to see Buffy or Dawn when Joyce died.
Because of his lack of support Buffy had to get the job at the burger joint.

Pretty sure Hank is not dead.

As for people remembering Dawn, I always took it as they inserted her "into the universe", meaning Dawn is remembered by everything and everyone in it.

...retroactive.

Ok, so they know that Dawn was the Key and that she was inserted... but does the gang still have these false memories? Or did their memories return to normal once they found out what she was?

They know now. I don’t remember any indication that their previous [inserted] memories were changed. In fact, I seem to recall a coupla times where Buffy or somebody else had a moment of confusion about the past that didn’t exist.

Now everything seems to be that “we” understand Dawn has been around all the time.

“So how, fer instance, did Dawn’s ‘transcripts’ make it to Sunnydale High?”

Yep... It's like a parallel universe thing... As of Season 5, she's always been there... So the events in Seasons 1-4 no longer happened exactly the way we saw them... Dawn was there too, and presumably has a history with all the characters and events...