 If you liked
Koisuru Boukun
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 ...then you might like
Sekaiichi Hatsukoi Movie: Yokozawa Takafumi no Baai
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Yaoi/shonen ai which has tsundere uke, somehow there is a child girl there, and there is a blackmail somehow too and a persistent seme somehow too, again. One is a movie and the other ovas but the duration is practically the same. And of course Koisuru Boukun is more explicit.