Originally posted by DarkL0rd10:

How do we know Noelle had a sister, how do we know that her name is December, d how do we know that Agore killed her, how do we know about that shadow cat thing, and how do we know Susie is (supposedly) homeless.

December was foreshadowed in the “spamton sweepstakes” event, and we see her room in chapter 4. Her name is given in chapter 2s berdly flashback (Noelle has a panic attack on stage and hesitates to spell the word). We don’t know that asgore is directly tied to her dissapearence, but we do know he left the police force at some point, and some event caused toriel to leave him. Given that there’s never any crime in this town (alphys mentions police are just for show) a child’s disappearance seems like the next most likely reason he’d be kicked off the force/resign out of shame. Susie was presumed to be homeless as she doesn’t make any attempt to call parents or return home at the end of chapter 2, but this was debunked in chapter 3 showing her room. Chapter 4 implies she has a bad home life (she doesn’t want to enter her house with Kris, and seeing toriel drunk reminded her of something clearly).

How do you *Not* know Noelle has a sister by this point? It was already greatly implied in chapter 2 but at this point but chapter 3 and 4 make it so obvious I'm not sure how you'd be unaware of that.
The Asgore thing is just a joke people made up, it's not real.
FRIEND's made quite a few appearances both in and out of the game so far. 1 in Queen's basement, during the cutscene at the start of chapter 3, during the Eram fight, in a dark room in the 3rd Sanctuary, during the Airwaves/Batttat minigame in the Mike room and on some of the hidden pages during the Spamton Sweepstakes. With some those appearances them either having cat ears (Chapter 3 stuff), having their sprite actually just be named "Cat" (rare cats Spamton Sweepstakes game page) along with their face appearing in the dark generally resembling the Cheshire Cat from Alice in Wonderland.
It was considered during Chapter 2 times with her not wanting to call her parents, the comments she made on her room in Castle Town and the way she seemed so comfortable eating things most people wouldn't call food (it was either that or she had neglectful/abusive parents which seems more likely now after Chapter 3/4, I can go into further detail on that bit if you'd like)