Here is the full list of all 22 Loser's Romance achievements.
Unlock all drawings (second-best ending rewards).
Unlock all senior pictures (best ending rewards).
Start a relationship with Gretchen.
Start a relationship with Padmaroopa.
Start a relationship with Tabitha.
Start a relationship with Wendy.
Start a relationship with Yang.
Visit Wendy's grandma without Wendy.
Meet everyone from your homeroom in the same run.
Wait, what's the problem?
Find the Panel Galaxy booth.
Successfully challenge your teacher.
Stay single.
If Gretchen Kantor lets you.
Give Wendy the ideal Christmas present.
Induce maximum cringe.
Help Tabitha make an animal friend.
Make it into someone's will.
Unlock all snapshots (best friendship ending rewards).
Unlock all wedding portraits (expert mode best ending rewards).
Start a relationship with Carrie.
Start a relationship with Keller.