This is Haguhagu's Sasha-chan and her classmate otaku vol 2 manga. it is NON-H. 162 pages. Looking for somebody to translate it. This is the 1988 pixels width, but I got the original files, so if somebody wants higher resolution, I can share them.

So does this series take place before or after Sasha makes her "debut"?

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Different universe all together. in this universe she never had a "debut".

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What are those few pages at the end, the ones that say chapter 0?

Is that the same senpai from the NTR short Hagu made?

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yes, it is the same senpai, he is also in vol 1. Sasha claims "he only comes over to chat" in vol 1 but it turns out she is secretly "going out" with him and it is implied they go to a love hotel and have sex. Sasha asks Fujiwara to keep it a secret that he sees them together from their classmates. Fujiwara says: "I caught a glimpse of the "adult world". on page 159. Hagu numbered it ch 00, it happens at the beginning of the story in April, was the first time Fujiwara was aware of a girl's beauty.
Also, the manga ends with the same senpai sending Sasha a message on page 153 while the Master is at his home, furiously drawing manga. The text on the side says: "to be continued in vol 3".

I was able to roughly translate most of the manga using Google translate and it did work Ok, except for a few pages. If I can't find a translator, I might do the translations myself.

On page 152, Sasha says: "I miss you papa, and I'm bored" in Russian. then her phone pings and a she gets a LIME message and she says: "Master"!, thinking it is Fujiwara who is sending her a message.

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正太什么时候变身肥猪？

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