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Tear Jerker: OFF
The Room shows the Guardians to all be hopeful individuals, who wish to create a better world after it apparently got destroyed. Knowing what they become by the time the game starts...
What's worse, you can see how they tried to retain their hopes of creating a better world for Hugo. Dedan promised Hugo pedalo rides, and his office in Alma is the only area of the zone that contains them. Not to mention his fondness of dates and timekeeping probably came from Hugo showing him what day it was from a calendar. Japhet keeps the book that he got on orchids in his library, which he regards as his greatest creation. Enoch wanted to give Hugo sweets, so he created his own element, sugar, to do so.
After beating Zone 2, you can find Judge wandering around the top of the library in the now purified Zone, meowing out mournfully for his brother Valerie.
"Have you seen my dear brother?"
Both the official and special endings.
And how about the credits music, huh? Somewhere Over the Rainbow by Judy Garland plays, from the famous Wizard of Oz.
The special ending also has an extra degree of soul-crushing if you stick around for the credits: you get to see the Judge wandering the three zones you purified, completely alone.
Having played the game to the end makes the battle with the Queen in any subsequent gameplays a depressing ordeal.
"You don't even know his first name"
A meta example, but someone asked Mortis Ghost if the cat mask that Zacharie wears after Valerie's death was made from his body. And what was his answer?
"No, Pablo, Valerie and Zacharie are best friends. He wouldn't do that to them."
Hugo's last words."I'm scared... of the dark...."
To which the Batter replies, "From now on, there will be no more darkness." Whether you find that bitterly reassuring or utterly callous and horrifying depends on how you view the story.
On the OST, you might find Japhet's Battle Theme Music less ominous and more mournful than the other two Guardians. The tearjerker part's at the end, when it hits a tape recording error sound then fades into cheerful birdsong and a bittersweet piano...knowing Japhet's backstory adds the impact.
This brief exchange after defeating the queen
Queen: Look... he has your eyes...
Batter: They are full of fear.
If you subscribe to the theory that Baby Hugo is the son of the Batter and the Queen, his death is extra tragic for several reasons. The world that we explore in Off is utterly artificial; Liquid water has been replaced by plastic and the land is all metal. The Elsens themselves are implied to be artificial as well, unable to reproduce and seemingly just there to mold the world back into something resembling what it once was. Hugo represents the first hope that humanity in some form will survive the calamity that took place before the beginning of the game, as he seems to have been naturally born.