Stalker

Stalkers were Byzantine assailants who were determined to kill the Assassin Ezio Auditore in the city of Constantinople during 1511. When they found Ezio in the city, they would sneak up behind him and attempt to stab him. However, they were often disarmed by Ezio and killed before they could stab him. Ezio could either ignore or kill the stalkers.

Stalkers were dressed similarly to civilians and wielded short blades, that they concealed in their sleeves before attacking.

Trivia

 * An appearance of a Stalker is indicated by a white knife on the mini-map, ambient whispering noise resembling the approach of your pursuer in multiplayer, and a change in the background music.
 * Similarly to the Cento Occhi thieves in Assassin's Creed: Brotherhood, Stalkers carry a large sum of money. When their corpses are looted, the player will receive above one thousand Akce and occasionally rare bomb ingredients.
 * Sometimes, stalkers come in groups, but they'll flee once the main stalker is either killed or is fleeing. The only way to see if there are other stalkers is to activate Eagle Sense right after a stalker attack. The stalkers will be highlighted in red.
 * You need to kill 5 stalkers before they stab you in order to complete the second part of the Romanies challenge. Note that either countering or simply killing them will count, as long as they don't stab you.
 * If you turn around just before a stalker gets close enough to attempt to backstab you, Ezio will forcefully push the stalker backwards and send him flying onto the floor. This does not kill the stalker, however, and they will flee if Ezio does not finish them.
 * Killing a stalker reduces Templar Awareness by 25%.
 * Stalkers are also one of the possible attacking Templar unit types in Den Defense.

Source

 * Assassin's Creed: Revelations