A Tachikoma is an AI walker/roller tank in the Ghost in the Shell universe, appearing only in the Ghost in the Shell: Stand Alone Complex sub-universe. They are best described as a "spider type multi leg/multi ped combat vehicle equipped with artificial intelligence," possessing the ability to act as both personnel transport units and mobile weapons platforms.
Tachikoma are the size of a small car, and are of a middle-blue color, with four "eyes" fitted on the surface of their bodies. Three are on the "head" and one is beneath the abdomen. Each eye has three pinholes, possibly for triangulation. These eyes sometimes become expressive in the typical anime style. Tachikoma are controlled by individual AIs, are capable of speech and generally exhibit a childish, curious, joyful and active personality, although they are consummate professionals in the field. They normally operate as independent units and receive orders from human agents, but they can also be directly piloted from a cockpit in their abdomen.
Tachikoma have four legs and two arms. They can move by walking, or they can drive at high speed by using the wheeled footpads on each of their four legs. They are apparently street-legal (presuming no legal exemption for Section 9). Each wheel appears to be angled and omnidirectional, allowing the Tachikomas to move in any direction with their holonomic drive system. Other abilities of the Tachikoma include jumping great distances, sticking to vertical or inverted surfaces, engaging a thermoptic camouflage mechanism, and grappling/rappelling using their adhesive string launchers. Tachikoma maintain control of their legs while using wheels to drive down a road, and shift their weight around turns. They can also roll briefly on to two legs while driving to avoid an obstacle or pass through a narrow space. To make balance easier, they can move their heavy abdomens with a ball joint. Their movements when walking and jumping were modelled on a jumping spider. Almost all of the Tachikomas' physical abilities are showcased in Episode 2 of Stand Alone Complex, "Runaway Evidence - Testation."
Standard Tachikoma equipment includes a 7.62x51mm light machine gun mounted in the right arm, a secondary weapon hardpoint in the "snout" (usually a 50mm grenade launcher, capable of launching both explosive and gas grenades, but which can be replaced by a six-barrelled 12.7x99mm Gatling gun), a universal cybernetic connector on an extensible, prehensile cable in the left arm, and a built-in thermoptic camouflage system.
Tachikoma's are gifted with very childlike AI, unexpected of a machine in a governmental special forces team, which gives rise to much of the comedy relief in Stand Alone Complex. Though they possess individual artificial intelligence, every night they are synchronized, so they start the next day with identical consciousnesses that are each the sum of their total collective experience & development. This leads to identity confusion, since each Tachikoma has the same memories.
It is explained in the last episode of the first season that it is their curiosity that lets them be different from each other. It is curiosity that saves a personality from dying when linked with others.
A notable paradox arises from this synchronization, however. Though the Tachikoma have identical memories, their personalities and opinions are distinct. During the Stand Alone Complex series, an episode is entirely devoted to discussions among them.
These separate personalities reveal three "main" Tachikoma. The first one considers itself Batou's personal Tachikoma, which has a personality of a hyperactive child. It is curious, inquisitive, and tends to get many "bright" ideas. It is special, given that he pampers it with natural oil and refuses to operate any other while on assignments. The second major Tachikoma (possibly favored by Major Kusanagi) is more logical, acting as the straight man to the first. The third Tachikoma is somewhat slower than the others and will at times have a difficult time keeping up with the other Tachikomas when discuss about such in-depth topics such as what it means to be "alive." There is also a fourth Tachikoma with a distinctive personality, who is a bookworm and an intellectual. Its body was taken apart during the experimentation incident, but its AI has presumably been saved for further analysis.
At one point in the series, while all but three of the Tachikomas are either locked away in a lab for study or dismantled for study, three surviving Tachikoma units abandon their civilian posts to assist the Major and Batou. This is the only time in the series where the tachikoma vary in appearance, as a few units have been disarmed for either study or to be sold off to civilian contracts. Batou's unit remained mainly unchanged, although it was given a set of decals to represent its new ownership, and its liquid wire launchers had been removed. The other two have been more extensively repainted and or modified, with the remaining laboratory unit's cockpit having been replaced by a much smaller diagnostic module, and the other contracted unit now bearing a construction color scheme. Only the blue tachikoma retained it's grenade launcher.
Tachikomas also have been theorized by Proto, near the end of the second season, to have ghosts, likely making them some of the most advanced AI in the world. While not necessarily making them the most complex or technologically capable AI, this does make them unique.