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..