Oh boy... which one? There are like a million of them in JTA, sometimes even the heroes take the mantle xD
I'll just discuss the big bads of the of the plot
[SPOILER BTW]
Summary
One of them is a literal 'faustian devil' that tricks people into deals and delivers a super twisted version of what they asked for, delighting in the agony of the people he tricks as he watches their greatest dreams crumble to dust knowing he has the right for their soul now, and will usually 'collect' by turning people into living puppets for demons he summons, and is trying to conquer the world with... Albeit he pales in comparison to the other ones
There is one who commited a genocide and flayed the one person who tried to stop him... who also happened to be his brother.
The final one being said flayed brother hellbent on destroying everything in the world due to the mortifying pain he is STILL enduring many milenia later.
Among these three, everyone else seems pretty chill hu?