If every village has an idiot, apparently so does every ship in Starfleet, and Reginald Barclay fills that role for the Enterprise-D. We first hear of him through complaints of other crew members about his ineptitude, tardiness, and lack of effort.
He wastes much of his time in the holodeck indulging in fantasies about other crew-members, particularly female ones, to escape from and cover his intense insecurity. He also included a prostitute in Alexander's western-themed holodeck program; Worf did not approve.
He is one of the few people in the Federation who fears using the transporters. This wasn't considered so terrible with characters like McCoy or Pulaski, but for Barclay it's treated as another sign of insecurity. It's even named "transporter phobia".
Once, his mind was given an intelligence boost by an alien probe. He was able to improve the Enterprise's warp drive to take the ship to the probe's home system. This was how the aliens, more aliens of the week called the Cytherians, studied other species. Barclay's intelligence returned to its normal level after the experience.
In spite of his lack of confidence, he attempts to self-diagnose any possible medical conditions he might have. One such attempt resulted in Dr. Crusher's prescribing a treatment that caused the crew of the Enterprise to "devolve" (yet another Star Trek abuse of evolution) into animal-like forms. Barclay transformed into a spider-like creature.