@RandomBuzziness, I just started a save in order to test having roommates.  I've had them before in the dorms and in one of the student housing houses, but since my Sim was concentrating on her degree and playing soccer, I didn't pay much attention to the roommates the, other than to note that some are messy, some are neat freaks, and some love to repair broken items.🙂 
In my new save I used the motherlode cheat to pay my bills so I can just concentrate on the roommate dynamics/shenanigans ! (I will just have my Sim paint, garden, and play chess.)  So, upon posting the add, I had 4 potential roomies show up, and 3 of those were Mermaids, even though I am in Willow Creek! 😃  The other potential roomie was Alex Moyer from the Tutorial.   I rejected one Mermaid because I know she has the Mean personality trait, and didn't want to chance her also having an undesirable roommate trait.  I accepted the other 2 Mermaids and Alex Moyer. 
So far the guy Mermaid is puzzling since he seemed to leave a few piles of trash, yet cleaned up the dishes, on several occasions!  He also seems to really like Monte Cristo sandwiches!  (Good thing I put in the ice cream maker so I can make that diet ice cream when my Sim gets fat from eating all the Monte Cristo sandwiches! 🙌 )  
One of the other Mermaids I had the option to tell complain about public displays of affection, but I never actually saw this happen yet, so we will see.  So far Alex hasn't done anything of note.  It will be interesting to find out! 🤨