Sheila

"Hello, and thank you for activating the M808V Main Battle tank. You may call me Sheila" 

-Shiela

Sheila, originally named Phyllis, is the vehicle-only AI behind the Blue Team's Scorpion tank built by an American automotive company and assembled in Mexico.

Creation
Sheila is a product of the military contractor that made the lowest bid, something she is ashamed of. She was built by an American automotive corporation and assembled in Mexico.

Deployment On Blood Gulch
Sheila first appeared in episode 3 with fellow Blue Team member Caboose. Ironically, he is the only Blue Team member that can drive Sheila, despite his lack of intelligence. She is friendly and cheerful, but, being a tank, she has a habit of blowing people up, including her own teammate Church. Sheila is bombed out of action by Sarge's dropship early in the first season, but is repaired by Tex several episodes later. Donut later disables her again by pitching a plasma grenade halfway across the canyon into her cockpit at the end of season 1.

Sheila shows some indications of liking Caboose early in season 1. However, in the middle of season 2, she falls in love with the Red Team's robot, Lopez. Before long, the two attempt to lead a robot revolution against the humans in response to perceived abuse that they have received: being constantly blown up or possessed by ghosts. This idea stemmed from Church's attempt to prevent them from doing so, in his endeavors to change the past, in which Lopez to create a vehicle which would later be stolen and used by O'Malley. After O'Malley kidnaps Lopez, however, she puts aside her differences in order to help the soldiers retrieve him.

Because Sheila cannot fit through the teleporter (due to the fact that her AI was hardwired into the tank by her manufacturers), she remains behind on Blood Gulch while the Reds and Blues leave to pursue O'Malley. After a brief encounter with a time-traveling Church, who whispers a plan to her, Sheila powers down and presumably remains alone as the sole inhabitant of Blood Gulch for the better part of a millennium. It is thought that she may be the source of the distress signal that the Reds hear in season 3, as Church's plan included the words "a thousand years". A deleted scene on the Season Four DVD confirms that she is ordered to send out the distress signal and disguises her voice. In addition, all of the Reds except Sarge see her drive past the Red Base in Blood Gulch upon their return. When Sarge demotes Simmons for his talk of "imaginary" tanks, she helps Simmons attempt his revenge on the Reds since she is unable to recognize the Red Team as her enemies due to faltering memory units. She also expresses dissatisfaction when Church later collects all the vehicles available to the Blue Base in Blood Gulch; Church perceives her reaction as jealousy (this scene was relocated to the Deleted Scenes section of the Season 4 DVD for continuity reasons).

Later in season 5, Church brought Sheila along to claim the dropship which had just landed but then Church noticed that Sheila was acting weirdly so in episode 85 he sent the worthless medic (Doc) along to try and help the tank calm down, however, it quickly became clear that doc's methods were not very effective on robots. Later, Doc reported back to Church and he concluded that something is wrong with her so Church decided to look for "somebody sneaky to turn her off" and they eventually decided to send Caboose so then they went to Tex to explain the plan and ask her to access the panel to shut Sheila down well Caboose distracts her. Then in episode 91 Caboose started distracting Sheila and he asked Sheila where the Omega AI is but then Tex shut Sheila down before Caboose could find out so in episode 92 Caboose revealed to Tex and Church that Sheila knew where Omega went to so they were looking for a place to put Sheila, and then Caboose found out that the Sheila AI can only be put in very similar vehicles, so they decided to move her into the dropship to try and find out the information about Omega.

Transfer To The Ship
In episode 95 Sheila was activated on the ship and she found out that she needed an ignition coil to become fully functional and she told Caboose that the blue leader was the one with the Omega AI and Caboose figured that it was Church when it was actually Captain Flowers who was occupied by Omega.

In Reconstruction, it is revealed that she survived the ship's crash, but sustained extreme damage in the process. Nonetheless, she is able to replay the recordings of the crash. It is also possible that damage she was refering to was hardware, not software damage, since she said that she was not functioning at full capacity when she was still inhabiting tank is Season 5 and she said similar thing in Reconstruction ("Operating at minimal capacity").

In the sponsor videos Washington says that Sheila is a "smart" tank, which are rare. This explains why the Blues in Rats Nest thought it was weird and not normal that Caboose talked to vehicles.

Caboose
Caboose first meets Sheila in episode 7 and he has been attached to her ever since. In season 2, Caboose felt jealous of Lopez for having a relationship with Sheila and Caboose in Reconstruction chapter 5 encountered Sheila after she crashed and he showed signs of his attachment to her AI when he said things like "she is not a program!"

Lopez
Lopez started his relationship with Sheila in episode 29 when he fixed her and they were immediately attracted to one another, sparking jealousy from Caboose that was increased because he was occupied by the Omega AI at the time. Lopez's relationship was delayed because Tex came back to possess Lopez's body to come after Omega in Caboose. In episode 36 Lopez and Sheila went off to make a robot army which was proposed by Church when he went back in time. In episode 38 O'Malley took Lopez as a hostage and then he and Sheila did not get much of a relationship after that.

Andy
Caboose tells the other Blues in Episode 54 that Andy has known Sheila from previous encounters, but gives no specific explanation. Caboose brings Andy back to the Gulch in season 4, but no interaction between Sheila and Andy has yet occurred to support or refute Caboose's claim. Rooster Teeth stated that Andy was originally to be Sheila's ex-boyfriend; however, the idea was pushed to Season 5, along with, according to Hullum, "a really interesting character trait related to that Andy's relationship with Sheila". .

Speculation
It has been speculated that Sheila is actually the AI Sigma or another AI, most likely Phi(Her original name being "Phyllis". However, this is unlikely because Sheila stated she was hardwired into her tank (a regular AI could simply be put in a storage unit or a disk) and could not be easily removed. Although, there has been no proof of Sheila being a vehicle-only AI. She, just like Gamma, may have only been in vehicle form for a temporary time (it could have been a hiding place by command to protect her from The Meta, or to hide her like Church).