Kevin (When Worlds Collide)

Kevin was a robotic copy of a Tyrannosaurus rex who was briefly a companion of the Eleventh Doctor, along with Amy Pond and Rory Williams. Kevin found pretending to be a T-Rex unsettling, so he asked to travel with the Doctor so he could find a better place for himself.

First adventure with the Doctor
Kevin was created as an animatronic amusement for the prehistoric planet setting of a vast resort complex. Possessed of an individual personality, he considered himself more of an actor than a simple automaton and was often concerned with attaining the proper "motivation" to portray his role effectively. When an invading Sontaran fleet caused a temporal rift to form in the complex, Kevin's T-Rex programming overcame him. He began attacking everything and everyone in sight, including the Doctor. His personality was eventually restored and he helped the Doctor restore order to the complex, allowing Rory to ride him as a mount to defeat the Sontarans.

Disturbed by the experience of momentarily thinking like a real T-Rex, Kevin asked the Doctor if he could travel with him in hopes of finding a better role in life. The Doctor agreed and somehow got Kevin into the TARDIS. (COMIC: When Worlds Collide)

Life as a companion
Kevin's size and appearance occasionally caused difficulty in his travels with the Doctor. Unlike the Doctor's humanoid companions, Kevin could not blend in easily with many of the societies visited by the TARDIS crew. In his first week aboard the TARDIS, his size hindered a quick escape from Sitting Bull's warriors. He later tried and failed to impersonate a detective in 20th century Los Angeles. Kevin's tiny arms also caused problems, making it impossible for him to assist the Doctor in piloting the TARDIS. On the positive side, he proved himself adept at scaring away hostile aliens, although Amy admitted that she missed the usual "running away lots" tactic normally employed by the TARDIS crew. Kevin almost caused a panic in Japan when the locals mistook him for a kaiju (a colossal monster in the vein of the famous "Godzilla") and thought he was going to destroy Tokyo. The forlorn robot eventually confessed his troubles to Rory, saying he felt he would never become something better as long as other people judged him by his appearance. Rory consoled him by admitting his own troubles in adjusting to TARDIS life. He reassured Kevin that one day he would find his purpose.

When the TARDIS visited a space station known for its Granberry Smoothies, Kevin elected to remain inside the TARDIS, but came to the rescue when his friends were threatened by possessed cultists. A mind-controlling space squid appeared outside the station and took over the minds of most of the inhabitants, but Kevin and the Doctor were both immune to its powers. After rescuing Amy and Rory from the cultists, the Doctor fashioned a battle suit for Kevin and gave the robotic T-Rex a concoction designed to knock out the cephalopod. Kevin engaged the squid in combat. Making good use of the extended mechanical arms on his battle suit, he defeated the squid and freed the population of the station. Kevin decided to remain on the station as its new chief of security. (COMIC: Space Squid)

Personality
Kevin was generally mild-mannered and polite. He did not enjoy portraying a T-Rex, describing the experience as wanting to kill people but not knowing why and not being able to stop himself. The robot T-Rex was also a capable fighter despite his friendly disposition. He allowed Rory to use him as a mount in combat and carried the Doctor to safety when they were being pursued by the cultists. (COMIC: Space Squid)

Behind the scenes

 * Kevin, perhaps owing to his oversized body, is never shown entering or exiting the TARDIS, although he did stick his head out the door to converse with the Doctor.