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.
Christine Chapel described her as a "mechanical geisha," doubtless meaning that Korby had constructed her as a sexual companion. (It may be notable that when Korby told her, "I couldn't love an android," Chapel responded not "Do you?," but "Did you?," perhaps subtly asking if he'd had intercourse with Andrea.) 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 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 the androids were vulnerable to unprogrammed emotions, for during this kiss, Korby activated a phaser, destroying himself and Andrea. (TOS: "What Are Little Girls Made Of?")