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I remember it's a reincarnation manhwa where an orphan girl gets adopted by a black-haired duke who has a son around 10 years old, but the king delays the adoption papers because he’s jealous that the duke is getting a daughter and wants to adopt her too?

Guys, can you help me find this manga? It's a BL manga where the seme is a detective and has an assistant who's into cosplay. The only thing I remember is that one of the cases they solved involved a cheating spouse (I think?). Huhuhu, I really forgot the title

Lim Hyeonseon has lived an ordinary life, and now, as he reaches his forties, he feels an urge to st...