This is the big villain of the first arc of the story, "THEIR DARK DESIGNS." The gist is that he was a master criminal, who operated around the world, and is an urban legend among thieves and criminals.
They called him the greatest criminal mastermind of the 20th century. His identity was a mystery. He would create "Puppets" of people he could control and kill remotely to perform his crimes, and never get caught. He'd be old… In his 60s when he first came to Gotham, but he is a very in-shape 60-year old.
In our story, we'll learn that years ago he recruited Joker, Penguin, Riddler, and Catwoman to help him perform his big final act. The biggest heist in all of history. But when they realized he planned to kill them, they killed him first. And then he starts to make his play in the present day, and the Bat-villains need to figure out how the hell that's possible.
In terms of his look, I'm going to throw a lot of contradictory ideas at you and see what you come back with… I'm more trying to evoke the spirit of the character. You should take all of it as inspiration, and see what you cook up.
If they did a Metal Gear Solid x Final Fantasy x Silent Hill crossover game, and the villain of that game was meant to be The actual, honest-to-god Devil… That's the kind of character I'm looking to create here. This should be a character with some complicated costume elements (to counter the simplicity of the Gotham Villains). He is not simple, he is ornate. And he should be scary.
When I picture him, he has a mask not unlike the mask of "Friend" from 20th Century Boys (seen below). A white cloth mask over his face (not a mummy bandage wrap like Hush). There should be a symbol on his face… Maybe an ornate gothic "D" in red.
His costume should look like an ornate general or emperor's military uniform that simultaneously looks like it's from a hundred years in the past and a hundred years in the future. Ornate armored shoulder plates… An ornate scarlet cape. Maybe some black feathers coming out from under the shoulder plates. He has a giant sword, but also a few guns in ornate holsters on his chest. A little bit Sephiroth from Final Fantasy VII, a little bit Edea from Final Fantasy VIII, a little bit Auron from Final Fantasy X… But with a final dash of Metal Gear Solid militarism, and a bit of horror.
The feel should be that THE DESIGNER is walking out of a different kind of comic book into Batman, a different aesthetic world entirely. There should be something eerie about The Joker, and The Penguin existing in the same space as this character. Something inherently WRONG about it. He is a different level of evil.
The key is for the first part of the year, regardless of the final reveal, is that this story should feel like Batman vs. The Devil.
"
. I told everybody about your S.H.I.E.L.D. mission to clear your name because all anybody saw on the news was Spider-Man robbing a bank. Matter fact, even when I thought you were a menace, you never tried to clear the air or make nice. You just trolled and antagonized me for years. And you should be glad I'm trying to clear your name because I'm the only one willing to do it. You have no other friends because you're a self-righteous a$$hole who thinks you have to do everything on your own. Even the Avengers don't fukk with you. 
"
. Yeah, he's right. fukk you, Spider-Man. 
