Louis Antoine de Saint-Just was a military and political leader of the French Revolution, a notable figure within the Jacobin club and one of the instigators of the Reign of Terror in the videogame Assassin's Creed: Unity. Saint-Just is the main antagonist of Coat of Arms and Up in Arms, two of the Paris Stories and the secondary antagonist of the murder mystery, The Body in the Brothel.
He's never seen in person in the game, though the database picture in his database entry details his appearance.
Assassin's Creed: Unity
The Body in the Brothel
Saint-Just saw an opportunity in arresting the infamous Marquis de Sade, who he saw as an immoral influence on the people of France. He then sent an agent to murder Suzanna, one of de Sade's favorite prostitutes and then put the blame on de Sade.
The agent saw that Olivier, de Sade's valet, had come to hate his master and the agent took profit of this by plotting the murder of Suzanna, hiring Olivier to kill her and place various items belonging to de Sade on the crime scene. Olivier did so, and the Assassin Arno Dorian was sent by the police to investigate the crime. Though all leads led to de Sade being the killer, Arno found a letter from Saint-Just that revealed that Olivier was the murderer and Saint-Just the man behind it.
Though he couldn't go against Saint-Just due to the man's enormous influence, Arno managed to arrest Olivier and put an end to the plot against de Sade.
Up in Arms
Saint-Just and his friend, Maximilien de Robespierre, plotted to end Napoleon Bonaparte's career by putting defective arms on Bonaparte's squad. The plot was foiled by Arno Dorian, who found a letter from Saint-Just and sabotaged the defective arms.
Coat of Arms
Saint-Just's valet had hired a tailor to do a fine and expensive coat for Saint-Just, but the tailor refused to finish the coat after he saw something strange on the coat's material. The valet then asked for Arno's help in convincing the tailor, but Arno instead investigated the rough and strange material of the coat.
He went to a slaughterhouse from where the material came from, and discovered that the coat was made of human flesh. Deciding to bring Saint-Just to justice, he went to the valet and confronted him about the coat. The valet told Arno that he'd probably make a fine coat himself, and attempted to kill him, though Arno managed to kill the valet.
Downfall and death
With the eventual fall of the Jacobins in 1794, Saint-Just, Robespierre and most of the Jacobin leaders were executed on the guillotine after the Thermidorian reaction.