Even though your mystery villain is hidden for most of your novel, you need to have a deep understanding of the character. The villain is a pillar character along with your sleuth and the victim.

In a mystery, the villain is a hidden antagonist. Opposition characters may keep your sleuth from discovering the villain or obstruct their investigation, but the villain remains hidden until the final reveal.

You need to know your villain inside and out. Know their motivation, their quirks, their weaknesses, their strengths. That way you can choose what to show and what to hide until the reveal. Your storytelling job is ti hide and hint. You want your reader to say, ”I should have guessed.”