If you liked
Sayonara Piano Sonata
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...then you might like
Shigatsu wa Kimi no Uso
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These series have piano prodigies as their main characters, and are mostly told in the male lead's point of view. In Kimiuso, it's the guy who used to be a piano prodigy and has a problematic past, and the girl helps him to overcome his problem. In Sayonara, it's the opposite.
If you liked
Sun-Ken Rock
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...then you might like
Wolf Guy: Ookami no Monshou
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Sun-ken Rock is the lighter and more comedic version of Wolf Guy. Ken, the protagonist of Sun-ken Rock, resembles Haguro of Wolf Guy, but Ken is much less scary, although as muscular as Haguro. And as strong as him. Both are about gangs, more or less. Not to mention both have explicit scenes. And rape.
If you liked
Utsuro no Hako to Zero no Maria
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...then you might like
"Fushigi" Toriatsukaimasu: Tsukumodou Kottouten
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Similar theme: about a boy and a beautiful yet mysterious girl who fight people who misuse double-edged magical devices; boxes in Hakomari and relics in Tsukumodo. Both have dark atmosphere, plot twists, interesting character developments, etc.