I think the whole bat family is mysterious.

THE JOKER hands down. Joker is the most mysterious and bizarre character in DC as well as Marvel. The fact that his history is unknown to the reader makes him more scary… I would even say that Joker is scarier and mysterious than Batman!

I vote for Joker. With no proper backstory and the mysterious motives behind the maniac with a smile, he is hands down the most mysterious villain of the DC universe. But yes, with so much knowledge? Brainiac can also be considered mysterious in his ways.

The batman family aren’t that mysterious maybe you were thinking about Jason mostly. He did get killed by the Joker then resurrected which makes people go crazy. But the rest of the family are sweet, they have their own way of showing their love.

Joker for sure