This achievement is for getting Miss Tressabell to settle down with you. You do this by first talking to her at the places she spawns around the map. These are in the Saloon in Clintville, the Sheriff's Office in Clintville, Rancher Bob's Place, and the Oil Tycoon's mansion.
After that, meet her at her place near the entrance to the train tunnel Southwest of Clintville. There, she will tell you she wants to buy a house and she has half of the money and you need to get the other half. That means you need to get $1,500. Get the money by killing a bunch of bandits and picking up money around the map, or doing the Golden Buffalo quest and just run in circles until it shits enough money.
Now go back to Miss Tressabell's place and she will give you her $1,500. Take this to the bank and buy the house from the teller.
Return to Miss Tressabell again and she will now go with you to the Riverside House. Once you are inside, the achievement unlocks.