also: ほんボシ · ホンボシ
noun
real culprit; prime suspect
Police slang for the actual perpetrator of a crime, as opposed to a mere suspect. Used in investigative contexts.
刑事たちはついに本星を割り出した。
The detectives finally identified the real culprit.
あの男が本星だという確証はまだない。
There's still no solid proof that he's the prime suspect.
真犯人 is the standard, formal term for 'real culprit', while 本星 is slang used mainly by police or in crime-related contexts.
From 本 (real, main) + 星 (star, mark). The exact origin is uncertain, but it likely derives from police jargon where a 'star' or 'mark' indicates a target or suspect.