Lieutenant DePaul was a male Human Starfleet officer who lived during the mid-23rd century. He served in the command division aboard the USS Enterprise under Captain James T. Kirk, during a historic five-year mission that starship undertook. He served as both a relief navigator and a relief helmsman. (TOS: "Arena", "A Taste of Armageddon")
Lieutenant DePaul was played by Sean Kenney, who is better known as the disfigured Christopher Pike in "The Menagerie, Part I" and "The Menagerie, Part II". On the final day of the two-parter's production shoot, Gene Roddenberry approached Kenney and, much to the actor's surprise, offered him the re-occurring role of Lieutenant DePaul. "That's when Lt. DePaul was born," noted Kenney. Roddenberry's offer of the role was because he wanted to thank Kenney for how he had played Pike, having dealt with a lot of makeup. (Captain Pike Found Alive!, "Chapter Three: The Formula")
About a month thereafter, Sean Kenney found himself back on the Paramount lot, to portray Lieutenant DePaul in "Arena". (Star Trek Magazine issue 175, p. 75) The performer was extremely relieved by the differences between this role and his earlier part on Star Trek. "It was a real treat," he reminisced, regarding the experience of personifying DePaul. "I could speak, I had lines!" (Captain Pike Found Alive!, "Chapter Three: The Formula") Kenney was also pleased he was able to play DePaul while having his own natural dark hair (whereas his hair had been dyed for depicting Pike) and that he didn't have to wear restrictive makeup which prevented him from speaking to other people. Because he had had to don such intensive makeup for playing Pike, few people knew DePaul wasn't the actor's first Star Trek role. (Starlog, issue #113) "So, doing 'Arena'," recalled Kenney, "it was like my first time on set." McCoy actor DeForest Kelley, at one point, became interested in trying to figure out how Kenney had obtained the part and, years later, Kenney joked he would never forget Kelley staring at him. "I'm sure De was thinking I was a little young for the part," Kenney clarified. Kelley asked about the specifics of Kenney's casting as DePaul and was flabbergasted when one of the men from the makeup department revealed Kenney had been Pike. (Star Trek Magazine issue 175, p. 75)
The revised final draft script for "A Taste of Armageddon" called this character "Cheffski".
DePaul's reappearance as the relief helmsman in "A Taste of Armageddon" came about because Sulu actor George Takei was on temporary leave from the series, filming the movie The Green Berets. (Star Trek Magazine issue 175, p. 75; Captain Pike Found Alive!, "Chapter Three: The Formula") Sean Kenney very much enjoyed returning to work as DePaul in "A Taste of Armaggedon", in which he felt he had "a little higher profile" than in the other Star Trek episodes he participated in.  He recounted, "I said, 'Wow, I'm just going upward on this show.' That was a great opportunity." (Star Trek Magazine issue 175, p. 75) Kenney was initially unaware, however, that the helmsman's controls were dangerous, which he referred to as "the one thing Takei didn't tell me before he left." (Captain Pike Found Alive!, "Chapter Three: The Formula") "When I sat at the console and pressed the plastic buttons," the actor explained, "sometimes they would get so hot my fingers would [burn and] stick- to the melting plastic. This was before the days of low-voltage bulbs. I couldn't yell out – I would just have to pull my fingers off as quickly as I could and hope the camera didn't pick it up." (Starlog, issue #113) The pain caused to Kenney's fingers was evident by the end of the first day of shooting "A Taste of Armageddon". "I went home the first night and laid them in a pack of crushed ice," he said. "Once burned, never again." (Captain Pike Found Alive!, "Chapter Three: The Formula")
Despite the discomfort of operating the helm station, Sean Kenney relaxed into his task of reappearing as DePaul in "A Taste of Armageddon". "I had no idea it'd be my last time on the show," he noted. (Star Trek Magazine issue 175, p. 75) Just when Kenney felt everything was going smoothly, Gene Roddenberry confessed to him that DePaul was to be replaced with a new Russian character, which eventually turned out to be Chekov. Kenney considered the replacement "a great favor." "Thankfully I got out early," he related, "because many felt that the actors in Star Trek became household faces and their careers ended by being too type cast [....] I've had no regrets leaving the original series." (Captain Pike Found Alive!, "Chapter Four: Kicked Upstairs")
Following the production of "A Taste of Armageddon", however, DePaul was written into the first draft script of ultimately undeveloped episode "He Walked Among Us" (dated 25 July 1967). In that installment, he was intended to have dialogue and appear in multiple scenes.
DePaul was given the first name "Sean" in Star Trek II: Biographies.