A bald, rotund man of unknown age, having to be responsible for Beavis and Butt-head destroyed his mind: he commonly had a horrified expression, sweated & shook constantly, would keep stuttering and uttering "uhhhh!!" mid-sentence, and was taking unidentified pills. A divine being has stated he had hair and a happy persona before he met them, and thanks to timeline issues (see Trivia) that means they ruined him within weeks. Every so often he'd have a scheme or opportunity to get them out of school (and sometimes to injure them and, in "Wet Behind the Rears", simply humiliate them), but something would always ruin it - in "April Fools!" he was forced to not expel them after realizing they'd then spend even more time being annoying.
Daria was one of Highland's honor students, and in "Citizen Butt-head" McVicker gave her and the others pre-assigned questions to ask a visiting President Clinton; Daria would ignore this and ask her own question. As far as we can tell, he never found out she did this as he was primarily focused on the two buttmunches. "Boxing Daria" has her say she started "writing violent revenge fantasies just to get a reaction" at age 15 and her teachers got called into school - McVicker would have seen her parents about this.
He came to believe that either he or the demented duo were going to kill the others, and was ecstatic when he thought they'd died in "Beavis and Butt-head are Dead"... only to have a heart attack when they turned up again. Sadly for him, he did not die and still has to deal with them. Forever.
In the "Miserable Life" alternate universe, where Butt-head was never born, a happy, calm McVicker leads a Christmas caroler group that includes a happy, socially-involved Daria. They may have more interaction in this world.
"It's A Miserable Life" blamed Butt-head for baldness in the year 1995, when the show had been on for four years under a 'sliding timescale'. Daria,however, had chronological progression and appears take place across her tenth to twelfth grades, putting Beavis and Butt-head as her ninth grade... which means McVicker lost his hair and mind almost immediately after Beavis and Butt-head went to his school!
In the DVD feature "Taint of Greatness: The Story of Beavis and Butt-head Part 1", Mike Judge said McVicker's voice was based on a real person from his childhood.
He seems to have died at the end of "Beavis and Butt-head are Dead"; however, he's back to life again in the 2011 revival of the show. His final appearance is in the episode "School Test" where he is arrested for attempting to help Beavis and Butt-head with their standardized tests out of irritation.