From what I can gather from this quest, the previous ones in the list, and the functions and flavor text of the various quest rewards, I believe that I may have figured out the likely motivation of our mysterious quest giver.
Every quest we went through involved dealing manifestations of negative emotions, such as fear and loneliness, and at the end of each one we received a reward relating to each emotion. The first one gave us a weapon that used the disgust and disdain of the hero for the Rose's actions to increase our effectiveness in battle against the Rose, while allowing us to temper it into a form directed specifically at those who we engage in battle rather than the whole of the organization, as those are the ones most likely to be engaging in the actions we disagree with.
The second gave us a helm filled with the regrets and remorse we felt from our various failures, specifically those that ended up in others losing their loved ones. However, the helm also grants a massive END boost, suggesting that it also grants us the ability to push past that sadness and repurpose it into the motivation needed to keep going in the face of adversity for the sake of preventing anything like that from happening again.
After that was the quest where we faced the feeling of betrayal we felt after seeing both ourselves and those around us being turned on by those they trusted. At the end of the quest, we get a sheathed Rose weapon that acts as both a comforting presence, that makes us feel safe, as well as a force that constantly reminds us to be wary of everyone, even those we trust, as traitors (or in the case of Grams, imposters) could easily take advantage of the unaware.
Then we did the quest for both Fear and Loneliness (or solitude, as is the more specific translation). At the end of that quest we received a necklace that makes us braver, and yet when we gaze into it, we can see our own terrified reflection. This suggests that it is designed to help us push past our fears and stand up to horrifying enemies and situations, even when we'd normally be on the verge of fleeing in terror.
Finally, we get the final quest, where we find and receive a seemingly cursed doll that brings out these negative emotions in those who touch it. Not only that, but see that how they affect someone is tempered by their personal ability to cope with those feelings. The orphan, who likely struggles constantly with anger for those who refuse to help him, a feeling of betrayal whenever someone who is meant to assist the townsfolk snubs him, and other such feelings more easily handles having them brought to the forefront by the doll. On the other hand, the Rose Soldier, who likely pushes his regrets out of the way enough that he can act in a way that he explicitly mentions is antithetical to how he'd want his daughter to be treated, is quickly overtaken by his emotions and is nearly consumed by them, as shown by the complete darkening of his eyes, and eventually of his entire person. Eventually, by talking and finding a way to not just push past these emotions, but to use them as a basis on which they can better themselves, the two are freed from the doll's spell as happier, better people.
Because of this, I believe that the intentions of the quest giver was to use us to create a medium through which it could forcibly engineer situations where people would be forced to confront and deal with their inner darkness, the emotions and feelings they don't want to face, and either overcome or be consumed by them.
I don' really know why it would want this. Maybe it's a helpful figure who wants to help people better themselves, or maybe it's a malicious entity that wants to see people be consumed by their own darkness, but refuses to just force that to happen. Or maybe (and in my opinion more likely) it's a neutral being who simply wants to, as the quest form states, "Give them what they deserve," with those with a strong enough character being able to push through their darkness and earn a brighter future, while those who are weak being condemned to be swallowed by their despair.
Or maybe I'm just a crazy person who overthought this particular questline for some reason.
I probably don't.