https://www.dlsite.com/books/work/=/product_id/BJ229517.html
https://book.dmm.co.jp/detail/b164aisis01205/

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group: FeeedTL [email protected]
commissioner: espeon https://gender.tf

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If I'm interpreting it correctly, these two lines...

P.2 "He completely changed after 'that' incident and became super depressed." with a photo showing the dark-haired guy with his sister
P.14 "Have you gotten them back? Your real memories."

...imply that the dark-haired guy's sister was killed some time ago and made into the skinsuit by whoever owns that "workshop", and it changed him into her once he put it on. That's kinda creepy.

Base +34

It seems Yanaka kinda knows what's up, maybe he planned this.

Base +6

@meow_pao going off that and the other dialog it's looking like the blonde friend was dating his sister, but then she was turned into a skin (presumably by him). And now he's brought her brother to the cabin where the skin is kept, and he's now used the skin to bring her back to life by turning him into her. So, basically killing him to get back his girlfriend.

I'm not 100% sure if the narration at the beginning is the friend or the brother, though.

Base +12