As a Machias, Wall Eehto (Fairy Tail) can alter his own personality to better suit his current predicament; for example, while fighting Laxus Dreyar, he discarded his carefree persona and adopted that of an emotionless fighting machine.
Persona (Valkyrie Crusade) can change her own personality with each mask she wears.
Zofis (Zatch Bell!) has the ability to alter the personalities of humans before he puts them under his command.
When Lori (Zatch Bell!) reads the spell Zeruku her mamodo, Kolulu's sweet personality changes to a bloodthirsty one suited for battle.
With Belial's mind controlling magic Hakuryuu (Magi the Labyrinth of Magic) can remove aspects of people's personality such as fear and mercy...
...turning people into brutal berserkers.
The spread of the Zombie Tattoo (Marchen Awakens Romance) began to influence Alviss, making him yearn to be by Phantom's side for eternity.
The Rowdyruff Boys (The Powerpuff Girls), had their similar personalities altered by HIM after he resurrected them, making Brick the hotheaded leader, Boomer the loudmouth dimwit and Butch the borderline psychopath.
Franklin Richards (Marvel Comics)
Emma Frost (Marvel Comics)
Mesmero (Marvel Comics)
Discord (My Little Pony series)
The Joker (DC Comics) is stated to suffer from a type of "super-sanity", altering his personality each day; he can either be a vicious killer or a harmless clown, depending on which one benefits him the most.