Professor Maggie Walsh was a major antagonist in Buffy the Vampire Slayer and the creator of Adam. She is the overarching antagonist of Season 4 and a posthmous antagonist in season 5.
Maggie was an evil genius who worked with the Initiative, a military-based organisation designed to fight against demons and the supernatural (however they were prone to inhuman behavior): Professor Walsh was also using captured demons to harvest body parts for the creation of her "ultimate" life-form: a fusion of man, machine and demon Adam was to be the Initiative's crowning glory and Professor Walsh was instrumental in caring for him and making sure he was designed to her satisfaction: however Professor Walsh succeeded too well in her plans and upon activation Adam proceeded to brutally kill her.
Later Professor Maggie Walsh is revived by Adam as part of his plan to erase humanity, transforming her into a cyber-demon - however she was little more than a drone and more accurately resembled a zombie than a true demon: having almost no will of her own and existing simply to do as Adam commanded.
Prior to these events however Professor Walsh had already been planning on killing off Buffy, viewing her as a danger to the Initiative and also feeling threatened by Buffy's influence over Riley (who she seemed to view as a son of sorts) - this was one of the major turning points in her character that cemented her as an antagonist - prior to this she was a rather extreme individual but was not truly villainious.
- Maggie could be considered the "Little Bad" of Season 4, in that she is played up to be the main antagonist, only for Adam to usurp her as the true "Big Bad".
Buffy The Vampire Slayer Big Bads