Users become stronger, faster, more durable, etc. through fear of oneself and others, possibly unlocking abilities related to the affinity and enhancing the existing powers. Some users may be able draw sustenance from fear or even slow or stop aging.
Terunosuke Miyamoto (JoJo's Bizarre Adventure Part IV Diamonds are Unbreakable); via Enigma
Gerard Valkyrie (Bleach) gains strength based on miracles manifested from the fears, hopes, feelings, and desires of the masses. His opponents' fear of him being unbeatable allowed him to manifest it and become truly unstoppable.
Thesulac demon (Angel)
Parallax (DC Comics)
Sinestro Corp (DC Comics) feed on fear to empower themselves.
Yellow Lantern Ring (DC Comics)
Harvesters like Ian Stone (The Deaths of Ian Stone) can feed of the fears of humans or else they'll starve and get weaker.
Sadao Maou (Hataraku Maou-sama!) can access his demon form through the fear of humans.
The Serpent (Marvel Comics) feeds on the fear that his Worthy instill in humanity, all over the world, ...
... until the fear of all of humanity made him powerful enough to perform virtually any feat.
Horrificator (Miraculous Ladybug) grows bigger and stronger the more fear she stirs in others.
The Bye Bye Man (The Bye Bye Man)
Pitch Black (Rise of the Guardians) is strengthened by the fear of others.
Charlotte Linlin's (One Piece) effectiveness in absorbing life is dependent on the victim's fear, the stronger their fear the more vulnerable they are to having their life drained.