The World God Only Knows/Nightmare Fuel


 * Kodama-sensei! The guy has some sort of unhingable jaw, and when he opens it wide (usually to yell at Keima for playing games in class) it turns into a portal through which an unending tunnel of Kodama heads can be seen screaming into infinity.
 * Vintage executive Lune, introduced in Flag 152, where she's shown restraining her murderous impulses by stabbing herself instead of the one she wants to kill, and vowing that once New Hell is overthrown, she'll be sure to 'pay back' the ones who annoyed her. Then you get a sense of why she has bandages covering various parts of her body, and are left wondering how many people she's made similar vows to before. All the while, her emotions keep switching between dead-eyed stoicism, insane laughter, and murderous frustration.
 * She's more scary in how she's the second really serious villain the series has had right after Fiore, who is extremely mild when compared to Lune.