Responses
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Sorry... missed the "seme" part. Here you go: when demon Endo takes human from, he has a "kid's dick" (revealed in Vol. 3 Ch. 19): http://www.mangago.zone/read-manga/konya_mo_nemurenai/
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I am also looking for that manga. I haven read it for three years now I looking for it. If my memory is correct, the story begin with a boy who was so popular but his only problem that he had a small dick. There was there quite kid who help him out. I really can't remember so that why I am looking for it.
Yaoi manga where the seme has a small penis