Jinx from The Ghost and Molly McGee is a villain I should have seen coming much earlier. She's pretty much the Chairman's biggest admirer, wields a scythe (fitting since the Chairman has Grim Reaper vibes), and always had her own plans—it was clear she would eventually betray or go further. Ironically, she embodies the kind of unrestrained evil I thought the Chairman would display.
To explain: she's essentially the main lackey of the Ghost Council and second-in-command—basically Cinder to the Chairman's Salem (from RWBY). She's able to summon a scythe, possesses extreme fighting skills, and, compared to most other ghosts, has virtually no redeeming qualities. Any "nice" moments are fake. She's extremely intelligent, easily outsmarting Scratch, holding her own in battles, and even recruiting him back to misery. She shows no nonsense when "fixing" problems—like attempting to kill Molly, a child. While she has some lighter moments, she plunges the world into darker misery and is highly sadistic toward her enemies.
Continuing on, she acts exactly how I imagined the Chairman would: a sadistic evil spirit not unlike enemies from Danny Phantom. Ironically, similar to Dark Danny in terms of ruthless transformation and villainy. Unlike the Chairman, who never truly tried to kill anyone on-screen, Jinx doesn't hold back and actively attempts to murder Molly.
The episode "Jinx vs. The Human World" ends with the titular ghost getting trapped in the Chens' Phantom Canister alongside the vicious Sobgoblins (not Frightmares). Jinx was quite a piece of work, but if and when she gets out, how much sanity will be left of her? Just like Dark Danny.
Though again, whether or not she's a wraith like Scratch… well, who knows?