The Advisors are a mostly synthetic almost "Cyborg" lifeform. They appear to have life support systems and are expected to not need oxygen, due to Dr. Breen complaining about their homeworlds atmosphere cannot support human life. These "life support systems" may be used to survive in earths atmosphere. It is also suspected that they are frozen in some sort of cryogenic freezer untill they are needed, They are extremely lethal although rarely seen, They are almost a "reaper" to "finish the job" such as killing Eli Vance when the overwatch and Dr. Breen failed.
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