K-Series

The K-series are not one model of synth, but a sect of synth within the EMC. As soldiers they possess combat abilities well beyond their human counterparts, due to their naturally higher agility, reaction time, and carrying capacity. Though not always trusted by their human comrades, those who've served with them accredit their capabilities.
There was a time when the bulk of the EMC military was combat synth. A large portion of human and synth personnel departed the Earth during Exodus, going on to found Jupiter Faction. Then, during the Synth Revolt, the majority of synth combat personnel still serving in the EMC defected to the rebellion. After the success of the Revolt, the Corps banned the manufacturing of all military combat synth within the Central Alliance. However the synth still serving at the time were protected by the loyalty doctrine, one of the founding directives of the EMC. Today they are known as the K-series.
While a group of diverse synth in terms of physical design and combat focus, the K-series do share commonalities, including a humanoid form that keeps them compatible as infrantry. Under the EMC mandate, K-series are hardlocked. As such they cannot interface directly with digital networks. In other words: they still need a trigger to fire a gun. During the Revolt, EMC synth were painted red to avoid being identified as the enemy rebels by human personnel. It was K-76 who first dawned metallic horns to give his chassis the appearance of a red devil. Despised by synth-friendly factions and untrusted by the human administrated EMC, more K series gradually followed suit. At present just about every K-series synth has two horns, to signify their unity, and their indifference to the perception of others.
The term K-series took from an old nickname among EMC commanders: 'killing machines.' Given their effectiveness in combat, the K-series often take point on the vanguard, clearing the way and leaving the leftovers for their human comrades. They made a name for themselves during the Reclamation War. The initial assault by the EMC was a general failure, but the K-series perserved on the battlefield, outliving their organic counterparts to the frustration of JF forces. In a unified effort, K-series survivors from various fallen units committed to the initial assault plan to capture Callisto fortress, and prevailed.

"When all their enemies are dead and the the cannons have fallen silent, the K-series walk among the bodies of friend and foe alike, unable to distinguish the fallen. Devils stalking the battlefield; incapable of empathy."

-- Captured JF Personell

The primary cause of death for K-series over the years has been data corruption. The youngest K-series being as old as the Synth Revolt (over 300 years), and the oldest predating the founding of the EMC, all standing K-series are hardened battle veterans. In culmination with stress and trauma commonly present in wartime, many k-series suffer what free synth refer to as integrity loss. When a K-series recognize their presence on the battlefield as a hindrance, they commonly commit suicide.

"They saved me because it was their duty, not because they care to know me."

--EMC Infantry Soldier