WARNING: The following contains spoilers for Batman #115 and Batman Secret Files: Peacekeeper-01, on sale now from DC.
Jervis Tetch -- aka the Mad Hatter -- was introduced in Batman #49 by Bill Finger and Lew Sayre Schwartz. A delusional genius obsessed with Lewis Carroll's stories about Alice and Wonderland, the Mad Hatter designed a series of hats that could allow him to control the minds of others and attack their psyches. Beginning life as a relatively benign Golden and Silver Age villain, he became an increasingly unsettling figure during the Post-Crisis DC Universe -- becoming more twisted and dangerous as the years wore on.
Even though Mad Hatter himself hasn't played much of an active role in the events of Fear State, he's increasingly been shown to have a major impact on it. At the beginning of the event, it was revealed that Simon Saint had provided Mad Hatter technology to Scarecrow -- inadvertently giving him the ability to further experiment with it and utilize it as a way of targeting people's minds. But as revealed in Batman #115 (by Becky Cloonan, James Tynion IV, Michael Conrad, Bengal, Jorge Corona, and Jorge Jimenez) and Batman Secret Files: Peacekeeper-01 #1 (by James Tynion IV, Ed Brisson, and Joshua Hixon), Mad Hatter's effect on Gotham has had a serious effect on how things are playing out in Fear State.
In Batman #115, the Unsnanity Collective is further explored during their time within the underground kingdom of Queen Ivy. Their leader, Master Wyze, designed the loosely affiliated group to help overcome their traumas by erasing their memories and making new versions of themselves. As they proved to be easy scapegoats for the plans of the Magistrate to prove to Gotham why they're necessary. As Wyze explains to Queen Ivy, he's the only member who didn't go fully through the process, because his personal experience and knowledge of the machines that wipe away traumas are too valuable. This means he remembers when he was he was known as Dormouse, who designed technology alongside Jervis Tetch before he'd ever become the villainous Mad Hatter.
When the Hatter went down that villainous path, however, he actually tried to force Wyze to become his March Hare. But Wyze wanted nothing to do with his partner's newest venture, and instead only came to him in his role newfound role at Arkham. It was there Wyze saw firsthand how the world treated the people it deemed unwell and was inspired to work on the Unsanity Collective. Wyze saw the pain in his former partner and thought there might be a way for people to heal in a different way -- helping setting up the state Gotham finds itself in. But the damage the Mad Hatter imparted on Gotham has also been thoroughly detrimental to others, like Sean Mahoney.
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Mahoney was the prison guard stuck in Arkham Asylum on A-Day, being seriously wounded by the explosion (and becoming a publically lauded hero after saving two nurses in the process). Transformed into Peacekeeper-01 by the Magistrate program, Mahoney has been driven wild by the Scarecrow's intervention. Batman Secret Files: Peacekeeper-01 delve into his origins as the son of a former GCPD officer who was ejected from the force when Jim Gordon took over as commissioner. When he was a child, Mahoney ended up briefly under the control of the Mad Hatter -- but his furious father refused to believe he was truly under control and beat him for it. Years later, as a security guard, Mahoney took revenge on an arrested Mad Hatter and viciously beat him -- leaving himself on the verge of getting fired.
This was why Mahoney was still in Arkham when the explosion set off. He even set the asylum to blow, intending to destroy evidence of his crimes, meaning his injuries were largely his own fault. Mad Hatter's influence on Mahoney helped prime him to become the furious figure he is now, the perfect tool to be used by Scarecrow to spread further chaos through Gotham. Even if he isn't an active presence in Fear State, Mad Hatter has quietly had a powerful effect on how Gotham has been targeted during Fear State -- with his influence affecting events throughout the city.
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