Simulacra are robotic soldiers with human minds employed by both the IMC and Frontier Militia, among other organizations. While they bear superficial resemblance to the commonly-fielded BRD-01 Spectre, Simulacra are instead human minds uploaded into robotic bodies. A simulacrum can be considered a form of transhumanism.
A simulacrum is a digital backup of a select individual's memories, knowledge, and personality. It is unknown precisely how Simulacra are made, but their most common application among the IMC and Frontier Militia was to preserve a valued Pilot whose original body was destroyed or damaged beyond repair. A copy of their digitized consciousness is downloaded and installed into a man-sized robotic frame, allowing those Pilots to live on and continue fighting. Generally speaking, and presumably to avoid confusion, there is presumed to only ever be one simulacrum of a given person walking around at a time.
Due to their human personalities and to maintain vestiges of their past humanity, Simulacra will often decorate or customize their chassis with trinkets or clothing, despite not having a practical need for them. This can be seen in the raccoon-like ponytails serving as "hair" sported by Phase Shift Pilots or the hood and mask worn by Ash, for example. A Simulacrum will still typically identify themselves as masculine or feminine, as well, and this is reflected in their chassis - not only can they speak with a masculine or feminine voice, but also a "female" Simulacrum has a more pronounced bust.
Not everyone takes well to the reality of their situation when they find themselves to be a simulacrum. Some suffer delusions that they are still living their past, living life, occasionally snapping back to reality violently. They may keep their old habits or routines they once had in life. Sometimes, programming is put in place such that the simulacrum never realizes they are a simulacrum at all, that they are still a normal person with a normal body. Regardless, some make the transition better than others.
Becoming a simulacrum is a process not exclusive to Pilots. Corporations are known to create simulacra of their own agents. That said, even after the end of the Frontier War, simulacra can still find work and prestige. Some have taken to working as soldiers of fortune or even assassins. There has been some controversy surrounding the simulacrum programs of certain companies like Hammond Robotics, if not the entire idea itself, but details are scarce.
In Titanfall 2, Simulacra known as Specialists can be fought as enemies in the campaign. They typically wield support weapons such as the MGL or Mastiff, and will deploy Plasma Drones and Ticks until dead. They are not a major threat by themselves, instead acting as support units. Specialists bear resemblance to Phase Shift Pilots in the multiplayer, but do not use any Pilot abilities. Militia-aligned Simulacra also briefly appear aboard the MCS James MacAllan during the mission The Pilot's Gauntlet.
In Multiplayer, players can play as male or female Simulacra with the Stim and Phase Shift Pilot Tactical Abilities. During the Titanfall 2 Technical Test, Grapple Pilots were given the model of what is now the Phase Shift Pilot, but this was changed for final release.