This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Street Fighter III
Yun and Yang are the same age in every game they appear in. Thankfully, their appearances in Street Fighter Alpha 3 and Capcom vs. SNK 2 are non-canon, but even in their SFIV cameo, they look the same as ever. It really makes you squirm sometimes...
We don't know anything about Q, or if it's even a him, or if it's even human. He is a creepy looking figure in a trenchcoat with a seemingly metal expressionless face, he breaths heavily and grunts without talking in fights. Absolutely everything about him is unknown, and he could be anywhere at any time. Even Q himself seems to agree with everyone on himself, as one of his winquotes has him uttering what sounds like "I am...abomination..."
Twelve is overall quite creepy on his own, but what really takes the cake is the downright chilling voice he has while copying an opponent with the X-Copy Super Art... It must be heard to understand...
Oro's loss portrait◊. Most of the other fighters are bruised and obviously look in bad shape when they lose, but Oro looks more like a damn corpse, with his skinny body, gaping mouth, and pitch-black eyes that look disturbingly like empty eye sockets..
"You're dealin' with cold blooded killers and assassinators! They'll decapitate your body and leave ya for alligators!" It may be just a hard hitting lyric in a song set around standing tall, pushing forward, and fighting all the way through in a series where throwing down on the street is the focus, but look deep into the supplementary material. Sure, Shadowlaw's been around, kidnapping people, subjecting them to experiments, killing them and disposing them in gods only knows where, but then we hit the Illuminati, who are rooted moreso in a realisic light, tied with not only corporations and the government, but also that of the mafia, drug cartels, big gangs, and even historic civilizations to move everything around in society as they please since the beginnings of human sentience. Outside of the ring, and without anything like the animes and even the anime movie for II and Alpha, what on earth else have they done?