 If you liked
Liar Game
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 ...then you might like
Reinouryokusha Odagiri Kyouko no Uso
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Same author/artist. Both feature a brilliant (jerk with a heart of gold) protagonist who makes use of their observational skills and knowledge of human nature to deduce what has happened, as well as to manipulate the future outcome towards a happy ending. ROKnU is the more light-hearted of the two, with Odagiri solving minor, stand-alone mysteries, while in Liar Game, the characters are pitted against a large, scheming organisation.