WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 23, "Sunset," now streaming on Funimation, Hulu and Crunchyroll.
Move over Gabi, there's a new worst person in Attack on Titan and his name is Floch Forster. A former member of the Survey Corps and now the leader of the Jaegerists, Floch has come a long way since his introduction. The forming of the Jaegerists first seemed like an unintended side effect of Eren's attack on the Liberio Internment Zone, but now it's clear that this radical faction was always part of the plan. In the wake of the activation of the Rumbling, Floch and the Jaegerists rounded up the Anti-Marleyan Volunteers, and Floch revealed that he's been in on Eren's true plan from the start.
Floch Forster was first introduced as a member of the Survey Corps who miraculously survived Erwin Smith's charge against the Beast Titan during the battle of Shiganshina in Season 3. Floch hated Erwin for convincing the Scouts to sacrifice themselves without knowing what they were dying for and even considering killing his commander when he found him clinging to life in the rubble. Instead, Floch decided Erwin didn't deserve to get off so easy and brought him to Levi to be revived by the Titan Serum. Floch reasoned that only a devil like Erwin could ever save humanity inside the Walls, and he was openly against using the Titan Serum to save Armin instead.
Floch's current position as Eren's apparent Number Two is at complete odds with how he initially felt about Eren. At a ceremony honoring the Scouts who successfully reclaimed Wall Maria, Floch berated Eren and Mikasa for letting their emotions influence Levi's decision to save Armin. Floch argued that everyone has lost people they love and it's not fair that Eren and Mikasa put their personal feelings in front of what was best for humanity. Going as far as telling Armin to his face that he didn't deserve to be saved, Floch made it clear that he may be a coward but is still willing to fight to protect humanity within the Walls, no matter the cost.
After Eren declared war on the world by attacking the Liberio Internment Zone as a preemptive strike to protect Paradis Island, a faction named the Jaegerists quickly formed. The Jaegerists supported Eren's actions, reasoning that it was only a matter of time until the world united to destroy Paradis Island and that Eren should use the Founding Titan to protect them. Surprisingly, Floch became the most vocal supporter of Eren and the de facto leader of the Jaegerists. As it turned out, Floch not only knew about Eren's attack ahead of time, but he also knew about Eren's plan to betray Zeke and his Euthanasia Plan to instead use the Rumbling to destroy the world, 10 months before it actually happened.
Now that the Jaegerists have overthrown Paradis' military and Eren has activated the Rumbling, Floch has seized control of the Island. Despite Floch's hatred for Marley, he sounded an awful lot like a Marleyan when he offered the captured Anti-Marleyan Volunteers "honorary Eldian" status if they cooperated. When one of the Volunteers resisted, Floch violently executed him while claiming to be speaking on Eren's behalf. Floch believes his actions will unite the people who remain on Paradis Island, even in the wake of the countless deaths Eren caused when he activated the Rumbling.
Eren's villain status is becoming harder and harder to dispute now that it's been revealed that his closest ally is a lunatic who executes anyone who doesn't obey him and agrees that destroying the world is the best way to protect Paradis Island. There's no one left to oppose Floch, as Armin has left to find Connie, Mikasa is struggling to think for herself and Jean is beginning to be swayed by Floch's argument that the Eldians on Paradis Island are finally free. Floch has been in on Eren's plan from the start, and that plan has gone off without a hitch thus far -- meaning that Floch just might succeed in holding Paradis Island in the name of the Jaegerists while Eren destroys the world.