Dead Age

During the course of a playthrough of Dead Age, you'll be able to get everyone in game to join your camp minus three people (so you can get max nine of twelve characters in one run). Of the missing three, two will be from the "random pool" (listed below) and one will either be James or Mary (when you do the mission to save one, the mission to save the other disappears.

The list of random characters is as follows: Bill, Hank, Lydia, Jamal, Jennifer, Larissa, Liz

Larissa is one of these random characters, so is not guaranteed to be available as a camp follower. Should he show up, you have three chances for that to happen the first week of the game (Day 1 - 7), and another early in week two (Day 7-14). After that, only one more "random" character will show up late in the game (around Day 28 - Day 32). Therein, if you're going for this achievement specifically, if you don't see the Hank Larissa quest by Day 10 or so, restart your game.

Larissa
At some point, you may get a quest to rescue a group of survivors in Area 4 of Highway Night. Head out there, and you'll need to fight two waves of zombies. Do so, and Larissa will return to your camp. Soon after, Larissa will ask you out to a picnic. Agree, and agree to the location of the Woods Day. You'll head out with her into the Woods Day until you reach a lake. Stay positive with here during this time, and eventually you'll be attacked by zombies. Defeat them, and continue to stay positive with Larissa, and you'll unlock this achievement.

In addition, following the above instructions will make Larissa a romanceable option on Day 33.