"Observer" here refers to the species. For Observer as the staff position, see Staff
The Observers are members of a sapient species separate from the Test Subjects.
It is not known how many of them exist, but their limited set of designation codenamed suggests not many are active at any one time.
Observers are easily recognizable due to their pure white bodies, single large eye and Bioluminescence. Their entire bodies glow, and are bright enough to be reflected relatively clearly off of a security camera monitor.
Other physical characteristics are unknown at this time.