Zachary is a teenage boy seen hanging out with Quinn Morgendorffer and the Fashion Club in the episode "Monster." He has a long face, and shoulder-length wavy brown hair. It is unclear whether he is a student at Lawndale High, though circumstantial evidence seems to suggest that he is.
Zachary in Canon
Zachary's lone speaking appearance is in the episode "Monster", in which he gives Quinn and the Fashion Club a ride to the mall. Quinn treats him dismissively, referring to him as "guy" or "Aaron," and sending him off to buy her something from Cashman's as soon as they arrive at the mall. Earlier in the episode, Quinn points out that she was dating Zachary because her first choice, Larry, totaled his car, and "Zachary has his mother's car". He doesn't have a speaking role in any other episodes.
When she pushes him into getting her a present, she tells him to ask for Theresa at Junior Five and she'll help him find something. That seems to have backfired: MTV's flipbook of Quinn's admirers, "Love is a Way Too Many Splendored Thing", has Quinn state that "I hear he ran off with Theresa on [sic] Junior Five. Good riddance!". Sour grapes, Quinn?
Zachary in Fanfiction
Zachary makes an appearance in "We All Fall Down," Part I of "Turnabout Confusion" by Dennis, as one of the boys who Daria dates in her efforts to win the bet with Quinn. He's portrayed as more perceptive and less obnoxious than Corey, if no less horny. He tells the football team that Daria's change in appearance "has something to do with Quinn," and expresses the hope that he can move fast with Daria if he strikes "while the iron is hot." After being refused for a second date and some threats from Mack, he backs off and isn't seen in Part II.