A vtuber's roommate (rm in short) is not a real person, it refers to the beauty standard or ideal self of the vtuber, typically something they will never be. It's a toxic thought that lives rent-free in their mind like a freeloading roommate.
For example you could read "Kiara's rm is an idol" (a particular type of Japanese singer) It means Kiara's ideal self-image is that of an idol singer, something she will never be, therefore she will keep pretending to sing and dance even though she's not very good at either, and nothing chat says will sway her away from it, because she has that toxic roommate thought.
Or you could read "Fauna's roommate is married" It means she sees herself as being married (like many girls do) but she won't be, not for a while anyways, because it's incompatible with her job as a vtuber, therefore she will always have a certain melancholy in her character, related to family and motherhood, which you can see even in her fans, like in the way they post UUUUUUUUUUUU... <crying emoji>
Hope this helps
