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A few clues he pieced together.
A pastry he gave neutaaa.
Behaviors when they encountered each other.
neutaaa's new look.
Login/Log off moments.
Voice chat.
Yeah, neutaaa talks about all their moments to the guild and Jigu figured it out. That's why he asked dude to reply to Jigbyul's message on his phone.
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I've read a crappy mtl... the normal translation is here but it's on 64th chapter (manhwa is on like 57th plot-wise I think??) https://wooksteahouse.com/variant/the-guild-member-next-door/
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I don't recommend reading the crappy mtl I've wolfed down, better wait for normal translation here https://wooksteahouse.com/variant/the-guild-member-next-door/
How did jigu learn that he is his neighbor? When??