Partner - Definition
"18. Your partner will now follow you back to older towns."
Found out that the definition of "Partner" is the special person with whom you are "living together".
Can also confirm that this works (even on old migrated saves). The person you live with will move with you wherever you go!
(Not tested what happens with old saves if you: lived with a partner in some town > moved away to a old town leaving your partner behind > then come back with another partner to that very town.
Sry im too lazy to test this as it is not relevant to new saves/most saves anyway.)