the letter says :
"My daily life has change because of Miyabi sisters. I can no longer laugh and smile like I used to. I am suffering alone it's so painful. Somebody save me."
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I am sure bro is being smitten that his life change, He no longer can laugh nor smile because how lovestuck he was. He is suffering because He likes at least 1 of 4 of them, Can He dating 4 of them at same time and become harem king?
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Nah just kidding, that letter is conspiracy, I bet Urara is suspect, scheming to bother quadruplets. She use girl An-chan as pawn