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 and beyond their human counterparts due to their higher agility, reaction time, and carrying capacity. Though not always trusted by their human comrades, those who've fight them highly rate their ability.
  There was a time when the bulk of the EMC combat roles were filled by synth. During the Exodus to found Jupiter Faction, a large portion of human and synth personnel departed the homeworld. Centuries later the Synth Revolt saw defection from the majority of synth combat personnel still serving in the EMC. After the success of the Revolt, the Corps banned the manufacturing of all military combat synth within the Central Alliance. However all synth still serving remain 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 design and combat focus, the K-series do share commonalities, including a humanoid form that keeps them compatible as infantry. 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 loyal 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 remaining K-series has some demonic accent, signifying their unity, and an indifference to the perception of others.

"When all their enemies are dead and the the cannons fall silent, the K-series walk among the bodies of friend and foe alike, indifferent to the distinction. Devils stalking the battlefield; incapable of a soul."

-- Captured JF Personell

The term K-series took from an popular nickname among EMC commanders: 'killing machines.' The K-series often take point on the vanguard, clearing the way and leaving crippled leftovers for their human comrades. They made a name for themselves during the Reclamation War. Where the initial assault by the EMC was a general failure, the K-series persevered 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 the Callisto fortress, and prevailed.
  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 (800 years), 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.