Andrea was a female android built sometime between 2261 and 2266, by Roger Korby. By Human standards, her behavior was quite simplistic, to the point where she would duplicate patterns of learned behavior.
When discussing Andrea's existence with Korby, Christine Chapel described her as a "mechanical geisha. To demonstrate that she was merely a machine and unable to feel emotions, Korby ordered her to kiss and then strike Captain Kirk. Later, when Kirk kissed her again, she next attempted to strike him, as she had done earlier. This time, he kissed her longer and much more passionately, and she became confused and upset, claiming she was "not programmed" for Kirk. This evidence that the androids could learn to feel emotions they were not programmed to feel contradicted Korby's belief that an android society would be a utopia, free of conflict and strife. It also provided Kirk with insights that enabled him to undermine Korby's control of Ruk.
Ordered to protect the complex after Ruk's destruction, Andrea mistakenly destroyed the android copy of Kirk. When she found Korby and realized her mistake, she became emotional. Perhaps threatened by his love for Christine Chapel, perhaps seeking his approval, Andrea began to kiss Korby passionately. This may have convinced Korby that Kirk was right, and that the androids were vulnerable to unprogrammed emotions, for during this kiss, Korby activated a laser pistol, destroying himself and Andrea. (TOS: "What Are Little Girls Made Of?")
Background information Edit
Andrea was portrayed by Sherry Jackson.
In the revised final draft script of "What Are Little Girls Made Of?", Andrea was physically described as "pale, dark-haired, lovely... even exquisite." She was also scripted to be extremely feminine. 
The reference book Star Trek: The Original Series 365 (p. 066) speculates that the character of Andrea may be closely related to the title of the episode she appeared in, "What Are Little Girls Made Of?". The book makes this connection due not only to the fact she is "indeed" a girl, albeit a girl android, but also because "one has to assume that all of the 'everything nice' ingredients went into creating her," a reference to "What Are Little Boys Made Of?", a nursery rhyme which provided the episode title.
Andrea's dress was designed by TOS Costume Designer William Ware Theiss. He evidently designed it to accent the natural contours of her body. (The Star Trek Compendium, 4th ed., p. 41) Star Trek: The Original Series 365 (p. 069) suggests that this costume might have been inspired by the work of designer Rudi Gernreich, particularly the monokini.