Cassette Beasts

As far as I'm aware, this can be achieved with any of the partner characters. Upon reaching 5 stars in a relationship, but before acknowledgement of such, a conversation will take place between the player character and the partner eluding to romantic ventures (in my playthrough, a movie night with Meredith). During this conversation, be sure to choose the dialogue option(s) that speak of dating, cuddling, kissing, that kind of thing

After this, keep them as your companion. Upon resting at a campfire, look for a pink dialogue option. Selecting this unlocks the achievement. Be aware you can only have one romantic partner per playthrough

Don't panic if the conversation where a pink option is available doesn't trigger the next time you rest; it takes a little. Treat it almost like a sixth heart in terms of time/actions taken