ARC

ARC (Autonomous Reasoning Center) was the mind of the Entity.

Imprisonment by SERVEYOUinc
For years, ARC was kept on the SERVEYOUinc-operated United System Research Base, where he was studied and tortured by Dr Ballard. (COMIC: The Sound of Our Voices) It managed to get out and starting reading the minds of the station personnel, which sent them into a coma. Slowly ARC started learning to communicate verbally. (COMIC: Whodunnit?) The Eleventh Doctor recognised these attempts and helped ARC switch completely. After that ARC restored all comatose people to their original state. The Doctor helped ARC escape from his captors and took him to travel in the TARDIS with Alice Obiefune and John Jones. (COMIC: The Sound of Our Voices)

Travels with the Doctor
Following a distress call, the TARDIS crew picked up the sole "survivor" from the planet Datastore 8, a Nimon who in reality was responsible for the planet's destruction. Although ARC was not strong enough to overpower the Nimon physically, ARC was instrumental in defeating him because Nimon could not absorb all of ARC's energy and exploded in the process. (COMIC: Space in Dimension Relative and Time)

ARC accompanied the Doctor to the Great Wheel, the mothership of the Amstron race. He felt the Talent Scout approaching and got scared, (COMIC: The Infinite Astronaut) which caused him to become prone to violence. This violence saved the Doctor and Jones from Amstron guards but made it harder for the Doctor to persuade ARC to let them go afterwards. When the Talent Scout arrived disguised as Alice's late mother Ada, (COMIC: The Eternal Dogfight) fear has become so unbearable for ARC that he contracted itself into a ball and remained in this state even after the Talent Scout teleported away. (COMIC: The Infinite Astronaut)

Reunion with the Entity
to be added

Alternate timeline
In a timeline later averted by the Doctor, ARC volunteered to take the Doctor's place to defeat the Nimon who infiltrated the TARDIS, despite potential risks to ARC's life. In this version of events, ARC assumed the Doctor's appearance, caused the Nimon to explode as in the final timeline and succeeded in getting rid of a black hole bomb created by the Nimon. (COMIC: Space in Dimension Relative and Time)

Biology and abilities
ARC's body did not have cells. The Eleventh Doctor described it as a big glob of stuff. (COMIC: The Infinite Astronaut)

ARC's extremities could significantly elongate. (COMIC: The Sound of Our Voices) ARC could also assume the appearance of another person but could not completely copy the way they speak. (COMIC: Space in Dimension Relative and Time) In addition, ARC could grow in size and semi-absorb other beings within his body rendering them immobile. (COMIC: The Eternal Dogfight)

By touching a person, ARC could transfer all information from their brain, leaving a perfectly healthy but comatose body behind. This process could be reversed. (COMIC: The Sound of Our Voices)

ARC's internal energy was so vast that even a Nimon could not handle it and would explode in an attempt to absorb all this energy. (COMIC: Space in Dimension Relative and Time)

Once ARC ate a chaise longue. (COMIC: The Eternal Dogfight)

ARC could feel when the Talent Scout was approaching. (COMIC: The Eternal Dogfight)

Behind the scenes
ARC is both a playable character and an enemy in the Doctor Who: Legacy mobile game.