The Head (backgrounder)
The Head has glasses even thicker than Daria's, completely obscuring his eyes; his vision must be extremely poor. He wears a haircut with long bangs that partially cover the right side of his face, and a T-shirt depicting the MTV cartoon character called The Head. Depending on your view of canon, this means he's either a fan of the in-universe TV show The Head or a fan of the "real-life" superhero called The Head. His fanon nickname comes from this T-shirt.
The Head in Canon
The Head only ever spoke in "Esteemsters," as a student in the Self-Esteem Class. When Mr. O'Neill asks the students what one means when talking about "ourselves," the Head tentatively says "we're...talking about us!" as if this was a startling revelation. Later, when Mr. O'Neill assigns them to think about how the world would be sadder without them, the Head asks if this means they'd never been born or "died suddenly and unexpectedly." He sounds disturbingly excited at this latter prospect.
At the assembly at the end of the episode, Daria and Jane are required to make a speech. The Head stands up and applauds before they even begin, showing apparent support for his fellow self-esteem sufferers.
After this episode he appears infrequently as a backgrounder. "Quinn the Brain" shows him sitting with the nerds at lunch, implying he's their friend. In "The Story of D" he is one of the boys who seem interested in the Fashion Club's newsletter. For the most part, though, he simply fills out the crowd.
A few odd appearances do exist, however. In both "Lane Miserables" and "Psycho Therapy," this character's face appeared on the side of milk cartons, a practice once done for children who had gone missing. No explanation for this is ever provided, as he is seen in school after both incidents. Also, in "Prize Fighters," he can be seen in two different places during the eating contest, demonstrating powers similar to Shaggy.
Unlike most background characters, "The Head" actually got an Alter Ego: himself as the astrological sign for Aquarius, similar to how fellow backgrounder Dawn was once shown as Libra. As another point of trivia, an MTV flipbook, which can be seen here, shows him performing in the school marching band. Presumably this was art intended for, but ultimately cut from, "I Loathe a Parade."
The Head in Fanfic
The Head has irregularly appeared in fanfiction as a minor background character, usually referred to by his nickname. He has yet to have a "breakout" appearance, though he did have a significant secondary role in TAG's "A Hard Days' Night," where he and several other backgrounders are part of a secret group of paranormal investigators known as the Irregulars. Within that story, his seldom-used real name is Derek.
He has a recurring role in the "Lawndale Fighting Championship" series under the name Ed "the Head" Parker. This name has since become common fanon. He also appears under that name in "Ethan Yeager in Lawndale," where he is Evan's tutor, and for a brief cameo in chapter 25 of "God Save The Esteem."