On day 32 you'll be able to choose which love interest you want, for this achievement you need to choose Lydia. This game involves a lot of RNG, so it's possible you never meet her. To get her you need Hank to show up one day and give the quest Hank's Better Half. You'll find her when you finish this quest. You'll be given Date with Lydia next, and once this is done she'll join your camp. Then choose her on day 32 and successfully complete the game to get this achievement.