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Guards ensure the security of the base by operating Security Consoles in Control Centers to identify and eliminate Intruders. They need Guns to effectively deal with Intruders (otherwise their usefulness is about the same as of normal colonists on Red Alert). Guards have to be allocated to your base in the Landing Permissions menu.
As of version 1.0.4, Guards have 20% armor, which results in 120% health. According to developers, this was a bug and has been fixed.