Major Motoko Kusanagi (aka Kusanagi Motoko-shousa aka 草薙素子少佐1) is the main character in Masamune Shirow's manga Ghost in the Shell. She is the leader of what is nominally an hostage-rescue unit acting under the authority of Public Peace Section Nine. It is mentioned early on in the original manga that Motoko Kusanagi is an alias, and in Man-Machine Interface she changes her name to Motoko Aramaki. That section of the manga also mentions that there may be more than one Motoko Kusanagi, though further evidence of this is not seen.
Major Kusanagi is a cyborg, with only a portion of her brain remaining from her original shell. Her body was built for military use, and she is generally faster, stronger, tougher, and more dexterous than any civilian cyborg or "unimproved" human. An interesting side effect of this body is that while she appears to be an attractive young woman who might weigh perhaps 130 pounds, she actually weights over 300.
In addition to her artificial body, her brain has been heavily augmented with computers, allowing her to interface directly with computerized devices, which in 2030 is more or less anything. These computers also improve her memory and thought processes. Her gray matter and associated transistor matter is in a removable casing which can be transferred into another body if needed, which comes in handy more often than you might think.
She is an extremely talented ghost hacker, meaning, essentially, that she does an excellent job of taking over, and messing with, people's brains. One trick she is very good at is "stealing someone's eyes"; you think you've killed her, but you've actually been shooting at empty space while she comes up behind you to send you into the next world. She is also adept at taking over the bodies of both androids and humans with computerized brains, using them for whatever purpose she feels like. These reasons have ranged from making a government minister punch himself in the face (she has a temper) to doing recon of a hostage situation by seeing though the eyes of one of the terrorists.
Prior to joining Section Nine, she led a freelance team that did black bag jobs for the government, typically ones so politically dangerous that no actual government unit could undertake them. She was maneuvered into bringing her team into Section Nine by Aramaki, the chief of Section Nine, but the history between them prior to this is not known. While her team is supposedly a hostage rescue unit, they typically engage in counter-intelligence and anti-terrorist actions, as well as less savory missions, such as the rather messy assassination of a foreign diplomat which starts off the original movie.
A series of events (which varies somewhat depending on the source) results in her merging with an artificial intelligence known as the Puppetmaster. This gives her access to substantial resources beyond her own formidable skills, but the full effects of this on her are not explored fully until Man Machine Interface. Shortly after merging with the Puppetmaster (minutes/hours in the movie, several months in the manga), she is shot by government forces, and her brain casing "disappears" with the help of her partner Batou.
Her personality is significantly more fleshed out in the manga than in the movie (where she appears calm and business-like nearly all the time). She can have quite a temper, and is very demanding of her subordinates. She is also prone to behaving erratically, such as an incident where she shoots and kills a suspected terrorist instead of capturing him as ordered. Such behavior may be related to her strong reliance on instinct and intuition when working a case; she bemoans her coworker Togusa's reliance on facts and logical deduction, feeling they are completely insufficient to handle the complex cases they deal with. Her representation in Stand Alone Complex feels much more true to the original manga: at one point her head is nearly crushed by a guy in an armored suit, and in return she comes within inches of killing him with a massive rifle provided by Saito. The great part of this scene is her palpable rage at being nearly killed; standing over him firing into his armor again and again until she is out of ammo, it's obvious that if the gun had been just a bit more powerful, her enemy would have been dead instead of ending up a prisoner.
In Man-Machine Interface, Motoko resurfaces under the name Motoko Aramaki, working as a security officer for a zaibatsu. She has completed the merger with the Puppetmaster, and her personality is significanly different. She is far less impulsive, and much less interested in getting involved in physical altercations than she was. Of course, she is also 5 years older by this point, so it's possible she's simply mellowed out a bit with age. But of anyone, fictional or real, she is about the last person I can imagine mellowing out.
She is extremely paranoid, and often appears controlling 'dolls', essentially remote-control androids, rather than risk her very expensive self. She has faked her death numerous times, often through the use of these tricks. Her only personal possesion of any note is a watch, which she has worn since she first became a cyborg, and has worn through the changing of many bodies. Many, if not most, people define themselves at least partially though their physical appearance. When your physical embodiment can be changed whenever it is convenient, that is a lot harder to hold on to. Apparently, Motoko uses this watch as a way of anchoring her sense of self.
There are few hints of what her personal life is like. One section of the manga shows her with a boyfriend (a member of Section One), whom she had been dating for around 6 months at the time; Batou makes a crack that this must be a new record for her. She also associates with three quite attractive young ladies in both the manga and in the series Stand Alone Complex, but their relationship is never made particularly clear. There are always various hints that Batou is interested in Motoko, most obviously in episode 25 of Stand Alone Complex; her response to these advances is typically of mild amusement.
The recent Stand Alone Complex TV series provides further (non-canonical) background information about a number of the major characters, including Kusanagi. She was involved in a serious plane crash at a young age, and was one of only two survivors. She was in a coma for months, and eventually the only way to save her life was to turn her into a full cyborg. It is confirmed in Man-Machine Interface that she has been a cyborg since a young girl, but does not state what event caused her to originally get the body. SAC also says that she was a special ops commando in the Japanese military before forming her team; she met Ishikawa, Batou, and Saito during this time period.
Motoko Kusanagi appears as a character in:
1: The Japanese term for major and the Unicode for Kusanagi's name were kindly provided by liveforever.