The Emotional Valley update introduced Joy and Sadness to Disney Dreamlight Valley, but while Joy is happy to move in right away, Sadness takes a little more convincing. Unlike a lot of character unlocks though, you technically only need to complete one quest before you can welcome Sadness. However, there's a lot of legwork to do in the run-up to triggering this quest which makes the process a little longer.
Before you can unlock Sadness as a villager, you need to have already completed all the quests to unlock Joy. This includes the Dream On quest, which gives you the item you need to take back to Sadness to start her unlock. Do this, and then follow these steps to welcome a new emotion to stay...
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How to unlock Sadness in Disney Dreamlight Valley
Source: PC Gamer (opens in a new tab)