The Scarecrow is Dr. Jonathan Crane. He is an enemy of Batman.
He is a mentally unstable former psychiatrist who uses a variety of drugs and psychological tactics to utilize the fears and phobias of his adversaries.
Crane at first works for Ra's al Ghul, developing and testing his fear toxin, capable of causing massive hallucinations based on one's worst fears.The fear toxin he uses is extracted from a mountaintop blue flower from China, and only works in vapour form. Crane also works to seemingly help captured mob henchmen for the powerful Falcone family, declaring the gangsters mentally unfit, thus allowing a transfer to Arkham Asylum, where Crane is able to continue his experiments on them.
If attacked, or to inspire fear at close quarters, Crane has developed two delivery dispersal systems for his toxin: a small, gas-dispensing canister mounted in his briefcase proves useful in the close confines of Arkham Asylum; if in need of more mobility, he uses a wrist-mounted version of the dispenser. To create an automatic fear response to the toxin, Crane often wears a burlap sack mask, crudely stitched together to resemble the head of a scarecrow. Although the mask seems non-threatening to behold, when used with the toxin, it creates a terrifying visage.
Crane's psychiatric abilities allow him to plan his moves carefully, weighing what he thinks others will do, and turning it to his advantage. His fierce, calculating intelligence also means that he invariably finds the idea of Batman amusing; if he could, Crane would give the Dark Knight a serious psychiatric analysis.
After Ra's al Ghul's defeat at the hands of Batman, Crane goes freelance, peddling his fear toxin to criminals for a fee. Although he is captured by Batman and arrested, it's unlikely that this diabolical master of fear will truly be stopped for long.
Crane reappears in Dark Knight Rises briefly. He's among the many inmates released from the Gotham in the massive riot to taking over the city. During the following months of 'Occupy New York' (I mean Occupy Gotham), Crane acts as a judge to sentence the rich and others who dare to fight the gangs, by allowing them to choice between death and exile.