Synth

In Half-Life 2: Raising the Bar, there is discussion of the "Synths" as results of enslaved races, and also mentions of the Combine as moving through the universe and enslaving and assimilating various races into their military forces and empire. This has led to the suggestion of two theories for the nature of the Synth: either they are techno-organic creatures genetically engineered by the Combine from the DNA of assimilated races, or they are fully-existing creatures in their own right that have been enslaved by the Combine as they are.

Various types of Synth have been brought over to Earth by the Combine, but other types of Synth are likely to exist. The total number of different Synth is unknown. It is likely that Synths were the primary forces used by the Combine during the Seven Hour War (this is likely, due to the newspaper clippings of the war that show a blurry synth Gunship), as the human-derived Combine units, as well as technology such as hunter-choppers and APCs, would have only started appearing after the Combine's rule on Earth had been established.

The design of the Combine Synths has a prominent shellfish design. The Valve offices have an aquarium including a nautilus, perhaps inadvertently being a source of inspiration for the Synth art design.