My Time At Portia

WARNING -- THIS IS MISSABLE
I know (as of the time of writing) it isn't flagged as such, but it is.

This achievement is awarded for completing Gust's romance questline. If you fail the final quest, or prevent it from triggering, you will not be able to get the achievement. I will say no more before the spoiler warning, but I will be explicit if you choose to read on.

Spoilers below -- I'm not going to bother flagging them since that would be basically the whole solution -- you've been warned.

So the achievement is awarded for completing Gust's romance questline. It is rather time-intensive, but not terribly difficult at all. If you don't want to marry Gust, simply reload your save from the morning of the final quest.

The first thing you will need to do is improve your relationship with Gust, which can be done by giving him gifts and playing/dating etc. The easiest way I found was through gifting two particular items:
- The book "Five Hundred Creative Designs" gives +15 points. You can get it from the game corner in the restaurant for 250 tickets. The dart game is quick and easy, but it is still a grind to get 250 tickets per day.
- Umbrella gives +10 points but is much easier to get in large quantities. You unlock the recipe early and they can be crafted from spines, copper, and fiber cloth -- all easy to get. If you need spines, you can kill urchines or put in a commission with the Civil Corps.
Once you get him to "Friend" status, you can Romance him by buying a Heart Knot from the flower shop. I had to wait until we were "Good Friends" before he actually would accept it.

Now with that out of the way, the romance questline can begin. There are 3 steps (technically more, but you get the achievement after the third):
(--EDIT-- Several people in the comments have said that they didn't have to do both A Date and Gust's Commission for the marriage quest to begin, so it's possible that if your relationship gets high enough, the aren't mandatory)
- A Date -- simple quest where you meet him at the Church at 19:00
- Gust's Commission -- a bit annoying, you have to take care of his pig for 6 days while he does other stuff. Make sure you feed the pig at least 5 times, or you will lose reputation when the quest ends. The pig likes Apples, Aroma Apples, and Milk, and it is "fed" by giving them as a gift.
- Commission of Happiness -- okay this is the achievement quest, and you must reach "Lover" status with Gust for it to trigger. Two important things to note: (1) You must have at least a level 2 house in order to be married, so the quest will not trigger until you upgrade your house at least once (2) This quest is timed -- if you don't complete it within the limit, you will fail and not get the achievement.
With that out of the way, on to the quest itself. You get a letter in your mailbox from Albert asking you do do a delivery for him. Talk to him and get 3 wallpapers to deliver. Deliver them before 18:00. Go back to Albert, and he will tell you to go talk to Gust on the beach. Do so immediately. Gust will then propose to you. You must accept his proposal to get the achievement. If you choose any of the other options, you will fail the mission and miss the achievement. I thought the answer about actually being in love with his sister was hilarious, so I picked it, he got pissed off, the mission failed, and I didn't get the achievement.

Other ways to miss this achievement:
- If you propose to Gust on your own, and he accepts, his proposal mission will not trigger.
- According to the game's Wiki (I did not verify this myself), if you're already engaged at the time he proposes, he will get mad, and the mission will fail.
There may be more ways to fail, but these (in addition to the others I mentioned in the body) I know of for sure.