Ethereal (Bureau)
Warning Spoilers Alert, Storyline Information from The Bureau
Beings of pure energy, without any physical substance. Homeworld unknown. Their appearance is of a tall, near-transaprent 4-armed humanoid, with a dozen thin tentacles coming from it's backside. The head is a large neckless dome that covers what is either a face, or a visible brain.
Two are encountered: a feminine-'voiced' electric-yellow one named Shamash, and an electric-blue one named Asaru, that speaks with Agent Carter's voice (though Shamash claims this is only because 'he' has not yet "know (his) own." She claims that they are beings of influence, and have merged with beings in multiple realities, with the intent of sharing information for the rare occasions that their kind meet. It is implied that they set themselves on planets that harbor the possibility of intelligent life, and simply wait for a suitable being to be born.
Shamash claims to be a much older Etherial, who long ago merged with the Zudjari scientist who would become Origin. Eventually, through years of study, Origin found a way to not also break their bond, but effectively enslave Shamash, creating the Mosaic network through her psionic abilities, and enslaving his own people, leading to the Outsider empire.
The other, Asaru, has been sleeping on Earth for an unknown amount of time. He was in hiberantion in the unseen artifact that was in the briefcase that Carter had transported to Area 51 (Groom Range), before Carter is attacked by the Sleepwalker, who then attempts to capture the artifact. When the Sleepwalker had near-fatally shot Carter and then began to strangle him, Asaru escapes and kills the sleepwalker and joins with Carter, and the player can now control Carter.
The implication is that the player is actually playing as Asaru (which explains the third-person aspect). When the Etherial bond is eventually shown, it is by the tentacles being linked to Carter's body and joints, suggesting puppetry. And as Director Faulke says, no other Agent or Squad has the success rate that Carter has: in truth, it is Asaru supplementing the abilities and directing both Carter and his squad with the precision of a computer (ie: a PC, a PS3 or an XBox 360).
Later, during the infiltration of the Outsider homeworld, Carter learns of the Etherial's existance from Origin, and manages to capture Shamash with the Venn Brace (impying that the Brace and the artifact are of similar nature), crippling the "Moz Zaaik" network. And with the detonation of a "Nuclelerium" bomb, Origin and the Mosaic core equipment (and much of the Outsider homeworld) are presumed to be destroyed. However, the years of study have given Origin enough knowledge to mimic an Etherial's abilities, and continue to exist as one within Mosaic (though at a severly weakened state, at a fraction of a true Etherial's power) in a backup core (that managed to escape through the Venn Gate that XCOM had used) on a Zudjari dreadnught, holding the remains of the network up, and able to continue the invasion, even assaulting XCOM's base itself, Site X.
After Shamash awakens in XCOM's labratory, Carter realizes that he had been controlled by Asaru (ie: you, the player), and destroys Shamash in a rage (while surprised that a handgun seemed to have worked in killing her), right at the start of the assault of Site X. Carter moves through the base while fighting Asaru's control, eventually threatening to set off a bomb to destroy them both, giving the player/Asaru about 30 seconds to release him. After ejecting from Carter, Asaru makes the choice of merging with either Director Faulke, Doctor Weir, or Agent Weaver; the three humans, realizing that the Etherials are telepathic, correctly assume that Asaru can 'hear' them, and give permission for him to merge with them.
With the new merger complete, Faulke, Wier, and Weaver escape Site X in the Avenger, and plan the assault on the faux-etherial Origin's mothership. The human that has merged with Asaru, having the ability of a full Etherial, states that if they connect with the core of the ship (the backup Mosaic Core), they can overwhelm and delete Origin for good.
When the Mosaic core is infiltrated (and while an infinite stream of regular and Elite Mutons swarm in), the linked human will use Asaru's power and give the order for all Outsiders to stand down, ending the invasion, and killing what's left of Origin. Depending on the human chosen, the Mosaic/Human/Etherial will command the remainding Outsiders through one of three endings, revealed as the human talks to The Council of their actions.
And when the Councilman asks of the "other creature," the human says that the Etherial bond was broken shortly after final operations, and that "its status, its location, and its intent are ... unknown." [1]
Link to Enemy Unknown
It is possible that these Ethereals are the Ethereals ones who managed to ascend, while the ones that appear in EU are the aliens that failed to ascend and so were forced to live in a physical body.