Batman villain. Obsessed with fear and it's effect on the mind.
Scarecrow (aka Jonathan Crane) first appeared in DC's World Finest Comics in 1941 as an antagonist for Batman, who had debuted only two years earlier. The character was created by Bill Finger and Batman's creator, Bob Kane. He was barely used for decades until a rebooted version was brought back in a 1967 Batman comic written by Gardner Fox with art by Sheldon Moloff. After this appearance, he became a recurring and popular character in the DC Universe.
He has also appeared in many of the Batman video-games and television shows. His live-action silver screen debut was in Batman Begins, where he was played by Cillian Murphy. Though he was in fact only a secondary villain, advertisers promoted him as a primary antagonist in order to make the character of Ra's al Ghul's reveal as a bad guy be a surprise. Scarecrow appeared in both of Batman Begin's sequels.
Jonathan Crane was head psychiatrist at Arkham Asylum, a psychiatric hospital for the criminally insane. Using information and funds from Ra's al Ghul, Crane developed a new drug that produced powerful and terrifying hallucinations that brought long-term effects such as paranoia, increased aggression, and schizophrenia. Crane tested his new drug, which could be released in gas form, on the most sadistic of the inmates and was pleased with the results.
Within a few months of Batman's first appearance, the caped crusader managed to take down Carmine Falcone, the most powerful mob boss in Gotham City. Falcone was not insane, but he was brought to Arkham Asylum for a standard psychiatric evaluation. The evaluation was administered behind closed doors by Crane. Crane, wearing a mask that resembled a Scarecrow's head in order to protect himself from the fumes, exposed Falcone to massive quantities of the gas. Falcone went insane and revealed all of the information he knew about the mob's inner workings.
Using the information taken form Falcone along with his fear gas, Crane became a powerful mob boss called Scarecrow. His private identity was not a secret to the inner circle of his gang, but it did not get back to the police and thus he retained his position as head psychiatrist at Arkham.
Batman raided Scarecrow's lair, but was incapacitated by the fear gas. Due to deus ex machina, Batman escaped and used his massive fortune and knowledge to test samples of the fear gas in his blood. Using this information, Batman invented an antidote.
In Batman's subsequent battle with Scarecrow, he easily defeated his foe. Batman then exposed Scarecrow to his own gas, driving Crane insane. Batman then revealed Crane's identity to the police. The Scarecrow, now mad, was locked up in the asylum he has once ran.
Ra's al Ghul used a more advanced version of the drug to drive much of Gotham insane. He then released the inmates of Arkham Asylum. Stealing a horse from a mounted policeman, Scarecrow road through Gotham raising havoc. He ran into Rachel Dawes, who was reporting on the corruption in Arkham Asylum. Scarecrow attacked Dawes, but she managed to get him off by tasering him. Scarecrow road off into the night screaming.
A year after his escape from Arkham, Scarecrow regained most of his sanity. However, Batman's vigilant efforts had made crime much harder and he was captured by Batman doing a risky weapons deal with Russian gangsters. He was nearly killed by a Batman impersonator, but Batman saved him.
Scarecrow was apparently not placed in an asylum in the events since The Dark Knight, or had been transferred out later. He had been jailed, but eight years later he was residing in a normal prison. Bane released the inmates and argued that they were imprisoned for being poor by a greedy and corrupt elite. After Bane showed evidence that Harvey Dent had become the villain Two-Face, most of Gotham City sided with him. Crane managed to get himself elected as "judge" over makeshift sentencings for the bougoise. Crane offered the "convicts"--who were all wealthy citizens or police officers--a choice between exile and death. Exile meant sending them out on thin ice where everyone drowned. Crane even sentenced Police Commissioner Gordon to "exile," but Batman managed to save him.
As Batman saved the city and order and law was restored once more, Scarecrow managed to avoid being killed by the police and remains at large. However, with a new Batman patrolling Gotham, it is likely he will not stay so for long.
Jonathan Crane has always been portrayed as a talented psychiatrist who had a high ranking position at Arkham Asylum. In most incarnations, exposure to his own fear gas causes him to go insane. This insanity is generally portrayed as paranoia, heightened aggression, and/or schizophrenia.