|Spoilers for The Bureau: XCOM Declassified follow.|
Asaru is a seemingly benevolent Ethereal, who glows a bright blue as opposed to Shamash's orange. This may be due to his young age in relation to Shamash or it may just be a different aura.
Asaru's still nascent form was hidden in the box that Carter was supposed to deliver to Director Faulke at the beginning of the game. When the Sleepwalker attempted to capture it and shot Carter, Asaru sensed Carter's fear and desperation and bonded to him to save his life. In doing so, he became "one" with Carter, subtly guiding his actions and even providing him with some of his skills, like Mind Control. William eventually discovers this, and refuses to be the slave of another creature, and tries in desperation, to get rid of him. He issues an ultimatum: Get out of his head, or he'll blow up himself, Asaru, and XCOM HQ with a bomb. Seeing no alternative, Asaru agrees and chooses one of three other possible hosts: Angela Weaver, Myron Faulke, or Dr. Alan Weir. Each will give a different ending.
It could be argued that Asaru IS actually the player character and Carter is his host and representative through most of the game, Asaru making most or all of Carter's decisions. After Carter rejects him, Asaru finds a new host and the PC will now play that new host, making Asaru the game's true protagonist.
In the Epilogue, the same tune from "XCOM: Enemy Unknown OST" is played when they reveal Asaru's location is unknown, fueling the speculation that Asaru (the player) has an unknown connection to the future of XCOM.