Guardian

Guardians
Homeworld	n/a
Skin color	Gray
Eye color	opalescent; glow silver when possessed directly
Distinctions	Husks of a race used by the Reapers to keep sentient life from venturing into dark space beyond the Alpha Relay
Average lifespan	unknown

[Source]

“Guardians have secrets to keep, and they are very jealous, haha!”

—Taephis Aesir, partially indoctrinated salarian.

“The Collectors were just the Reapers' scientists. These . . . these were their soldiers.”

—Alliance Rear Admiral Clayton Travers