Eugène François Vidocq (1775 – 1857) was a French criminalist who is often regarded as the father of criminology. Hailing from an early life of crime, his later prolific career as a policeman himself was an ironic twist of fate. His personal experience as a fugitive and the network he cultivated in the criminal underworld proved invaluable to the field of criminal investigation which he had a hand in inventing. He founded the detective unit that would become France's Sûreté.
Vidocq was born in 1775. In his youth, he was a petty thief who frequently found himself in jail. By the time of the French Revolution, he even relied on imprisonment to avoid starvation. 
Partnership with Arno Dorian
One day in either 1796 or 1797, he was imprisoned as usual at the jailhouse on the Île de la Cité when the Assassin Arno Dorian strode in. Arno offered the Minister of Police Charles Cochon de Lapparent his assistance in fighting crime. Although Lapparent declined, Vidocq revealed that the police chief had been unable to solve numerous murders and pointed out that Arno could solve the murders while Lapparent could receive the credit. The police chief was then persuaded to accept Arno's help.
When Lapparent later tried to sleep, Vidocq called Arno to his cell. He claimed that he could help the Assassin solve the murders because he possessed an intimate knowledge of the situation in the streets of Paris. Arno questioned what interest Vidocq had in solving murders, to which he responded that Lapparent was too lethargic when it came to resolving crime in the city, removing any hope of making Paris a better place to live. He claimed that he also wished to help in order to pass the time. From then on, Vidocq would give Arno instructions on how to solve murders properly and point him to cases across Paris throughout the duration of the revolution. In total, Vidocq aided Arno in solving fourteen murder cases and bringing their culprits to justice. Just prior to the conclusion of the fourteenth mission, he escaped from his cell, leading to another round on the run from the police.
This partnership with Arno foreshadowed Vidocq's eventual career as a policeman himself in a remarkable turnaround of his life. By the time Napoleon had risen to power, he had begun working as an informant for the police, who recognized that his connections to the underworld had much to offer in the way of intelligence. Notwithstanding this, his skills at evading the authorities, however unsuccessful from time to time, offered significant insight into crime-fighting techniques. Eventually, Vidocq formally joined the ranks of the emerging Parisian police force and became one of the forefathers of modern methods of criminal investigation. Even then, he continued to lead a double life on both sides of the law, as his criminal friends were constant assets to his career.
Behind the scenes
Eugène François Vidocq appears in Assassin's Creed: Unity (2014) as his younger self when he was still a petty criminal. He is first encountered in a jail cell at the police station run by Charles Cochon de Lapparent on the Île de la Cité. Meeting Lapparent and Vidocq here introduces the player, as Arno Dorian, to the Murder Mysteries line of side-missions. After completing all of the Murder Mysteries across France, Vidocq disappears from his jail cell with its door left ajar. Upon speaking to Lapparent about the street rat's whereabouts, the player is informed that delinquent escaped once again.