I'm mot sure why this happens. A butler is supposed to be attached to a single household. Every household that has a butler gets a different unique one. You can customize a Sim as a butler then release him/her back into NPC realm, (unplayed-out of world). You will need to add the butler hidden trait, of course. Usually when the next time a household hires a butler, that is who it will be. (You should do this right away) However the game does spawn random ones that you may have to delete a few times before you get the butler you want.
Another thing that may happen is another household that has a butler may lose that butler for whatever reason, so they steal the butler from the original household. I haven't tested this yet, but I would think go and fire the butler in the other household then hire one in the original and it should be the one you had for awhile. (If random ones appear, then as usual, just keep deleting them until he/she is back)
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I actually ended up getting a pretty cool random butler for the Goth household. :smiley:
https://i.imgur.com/qV2LG03.png
https://i.imgur.com/tEkKcTD.png
