Ted Buchanan was the central antagonist of the Buffy episode "Ted" - he was one of the first threats that Buffy faced that was more science-fiction based than supernatural, yet he was also one of her more challenging opponents (especially considering he was a one-shot "monster of the week").
He was portrayed by the late famous actor John Ritter.
Ted Buchanan, often referred to simply as Ted, comes across as a sweet natured gentleman and as such he wins the heart of Joyce Summers but not that of her daughter - Buffy Summers: Ted at first seems to act like a nice guy and tries to include Buffy into his new family as her potential new father, however when Buffy begins to rebel Ted starts to bully her and show a darker side that climaxes in the two engaging in a seemingly fatal battle in which Buffy uses her Slayer strength on Ted and kills him.
Shocked at what she had done Buffy tried to reason both with herself and her mother but was overcome with guilt, yet the nightmare was not over for her or Joyce as Ted Buchanon miraculously returned - very much alive: it was then his true nature was revealed, Ted was actually a deadly robot created long ago by a mad scientist who had a dislike of women, Ted seemed to inherit his creator's personality and would befriend women before murdering them if they "misbehaved".
Buffy fought Ted again to save herself and her mother from the murderous machine and managed to destroy Ted for good - ending his threat once and for all.