Basically papyrus and mettaton's first direct interaction (after the barrier is broken but before everyone heads to the surface, it's during a little period of time where people like Grillby, Muffet, Napstablook, etc. all show up and the UT squad decides to live together all under one roof) goes like this

~~~
Mettaton: (waves) Hello there sir! I believe I've heard of you! Oh, hello Frisk!

Frisk: Hi Met—

Papyrus: (SCOOPS THEM UP PROTECTIVELY) (DEMON VOICE) IF YOU DARE TO FUCKING TOUCH MY HUMAN I WILL TURN YOU INTO A FUCKING TOOTHBRUSH YOU BUCKET OF BOLTS GTFO

Mettaton: (tries to speak but no sound comes out) (RUNS FOR THE HILLS)
~~~

yeah papyrus is really salty for a long time because MTT tried to kill frisk for selfish as fuck reasons and was NOT happy when he had to live under the same roof as mettaton
MTT would never leave his room when Papyrus was home for a few days
Then eventually Alphys basically told him to stop being a pussy
which backfired and he got a baseball bat to the head in about thirty seconds and about thirty times

yeah
when papyrus says to get the fuck out
bitch you best get the fuck out

frisk did nothing to stop this since they like MTT but were still kinda disappointed in him for all the shitty things he's done
pap thought MTT was a shitty person until he got involved in charity work to make up for his mistakes, and then pap stopped cussing him out every time they were within 10 metres of each other and only hit him with the baseball bat like once or twice
eventually they became acquaintances and then pap tries to set him up with a date considering he's one of the only single people in the house

catty, muffet and shyren are MTT's favourites so far but paps is kinda concerned because catty is his childhood friend and he still doesn't 100% trust the robot to not break her heart
especially considering how selfish he still is