No one knows who the Joker really is. Little can be confirmed about his early life before he turns to a life of crime. Despite his capture, no traces can be found on his fingerprints, dental records or DNA matches against the GCPD's databases.
In a report filed by the GCPD, there were three theories presented for Joker's identity and origin. The first was that Joker was an escaped patient from Arkham Asylum. However, this theory is debunked on the basis that his identity has no basis within any records. The second theory was that Joker was a former employee of the Harley Brothers Circus, which was documented to have connections with the mob. It is reasonable to theorize that Joker is a disgruntled former employee that had bad business with the mob. The third theory presented was that Joker is a former soldier suffering from acute PTSD, explaining his cool demeanor and familiarity with weapons, as well as his devastating effectiveness in various forms of combat.