An alliance was formed.

So, I've talked about how Post-Pacifist Sans and Asgore are pretty chill friends. Sans probably taught him how to fist bump. They agreed that if they ever start driving trucks, their radio handles will be Big Mac and Tic Tac.

But how about Papyrus and Toriel?

...Truthfully, I don't think they'd get along as well on a personal level. They care deeply for each other, don't get me wrong. But I don't think they spend much of their free time with one another.

In my mind, their relationship is very... academic, for the most part. Like, I think Toriel views Papyrus similarly to how she would one of her students after they've graduated. She wants to guide him and teach him things, see him grow up to be a capable contributing member of society, dispense sagely life lessons, etc. Papyrus finds her motherly and comforting, and frequently asks her advice about things he doesn't know or understand... but thinks she's a little irritating and overbearing sometimes.

He has mixed feelings about her presence-- on one hand, when she and Sans get riled up into a pun session, it's doubly annoying and goes on for hours. On the other... she makes his brother happy, like, REALLY happy, and that's something no one besides himself has been able to do for a long time. Because of that he really pushes to have her around, despite not clicking with her. WHOOPSIE DOOPSIE WE ENDED UP AS YOUR NEXT DOOR NEIGHBORS ON THE SURFACE, NOW HOW DID THAT HAPPY ACCIDENT OCCUR??? HOW SERENDIPITOUS?!? *whistles while exiting through a window*

However, when they do team up, the results are pheonmenal. If they're both working towards the same goal, that goal gets done fast and done well. She's helped him a lot with his cooking skills, so when they both get to work in the kitchen on holidays, you're looking at a proper feast. Planning a party? You know they'll be up until 2 in the morning making the most bitchin' centerpieces you've ever seen. She often recruits him as a field trip chaperone for her classes: his blue magic and piercing voice makes herding small children a snap.

Usually those are all functional team ups. Not for fun. (They still high five after a job well done though. They're not barbarians.)

...But every once in a blue moon, they get mischievous. Pap is more often than not the victim of Sans' pranks, but sometimes Toriel feels like stepping in and declaring war, for no other reason than she's in the mood to take that fucker down. Papyrus isn't great at setting up pranks on his own, but Toriel is sharp as a tack and devious, downright diabolical, even! When the two of them get whispering and giggling together, Sans knows he's in for it.

She's not as annoying when she's on his side, Pap has to admit.

Papyrus, Toriel (c) Undertale, Toby Fox