Dib is the secondary antagonist of Invader Zim. He is Zim's arch-nemesis and is highly unpopular at school (where he shares a class with Zim) and at home for his broken record obsession with aliens, Bigfoot, the Loch Ness Monster, and other paranormal phenomena. Dib has been obsessed with the paranormal and supernatural since an early age. It is often misunderstood that Dib's surname is Membrane as his dad is called Professor Membrane. Membrane is actually the Professor's first name and it has never been explained what Dib's last name is.
Eric Trueheart, one of the show's writers, has stated in a commentary on the final Invader ZIM DVD that, if the series had not been canceled, a potential storyline might have included Dib discovering he was an artificial creation of Professor Membrane's making (similar to Steve Fox's storyline in Tekken 5).
In the episode Dark Harvest Dib mentions to his classmate, Torque Smacky that he has memories of being abducted as a baby "... Perhaps to create some kind of genius super baby?", though he also states his memory on this is unclear.
His obsession has made many of the people around him deem him crazy and/or stupid, including his classmates, his teacher Ms Bitters, his sister Gaz and his father, Professor Membrane. In the episode Gaz, Taster of Pork, Dib mentions that he may be obsessive-compulsive by saying: "How did I miss that? Me! The obsessive-compulsive Dib!"
Although he is a good person, Dib can be, however, extremely talkative and annoying, explaining himself constantly, which frustrates his sister Gaz to no end. When confronted with alternate versions of himself (such as in Future Dib and Dibship Rising) he can be aware of this.
Dib dreams of becoming a paranormal investigator when he grows up. He seems to regard this field as a legitimate one, though most people he knows do not; and, despite his fascination with many paranormal legends, he nevertheless is being shown critical of others, such as "Chickenfoot" (a supposed half-man, half-chicken that turned out to be a crazy man in a chicken suit) and a chocolate vampire, Count Cocofang (a parody of Count Chocula). He believes that paranormal study is as much about disproving frauds as pointing out the legitimate occurrences. (The Sad, Sad Tale of Chickenfoot , episode 18B)
Apart from Gaz, Dib is the only child at the "Skool" who realizes that Zim is an alien, and his sister doesn't seem to care. He is a member of the "Swollen Eyeball Network" conspiracy theory group, where he is known as Agent Mothman (a reference to the Mothman of West Virginian folklore). Prior to his discovery of Zim, Dib was already heavily involved in the paranormal, and people often throw his earlier ideas of 'ninja ghosts' and 'finding bigfoot in his garage' (where he was using the belt sander) back in his face when he asks for help.
Dib's hair is pulled back in a scythe-like manner, and he usually wears a black trench coat. A running gag in the show is the fact that everyone insults Dib for his supposedly-large head, although his head is more or less equal in proportion to the other characters. This joke began in the episode Door to Door, and has occurred in most of the future episodes.