ruffian; villain; knave; thief; suspicious fellow
peculiar person; idiosyncratic person; stubborn fellow
tricky thing; something that is more than it seems
expert; master; highly skilled person
goblin; apparition; monster; ghost; phantom; spectre; specter
あの問題は本当に曲者だったよ。
That question was a real tricky one.
警備員が倉庫の近くに曲者を見つけた。
The security guard spotted a suspicious fellow near the warehouse.
昨夜の事件には曲者が関わっていたらしい。
It seems a suspicious character was involved in last night's incident.