"Chief O'Brien would be proud to have him on his crew."
"I can't think of higher praise."

Crewman Kellin was a male Human officer in the 24th century Starfleet.

Kellin served during the Dominion War in 2375. He was assigned to AR-558, a planet that the Federation Alliance had captured from the Dominion, under the command of Captain Loomis. They were assigned to protect the planet's communication array. Other members of the 150-person unit included Commander Parker, Vargas, Reese, McGreevey and Lieutenant Nadia Larkin.

The team would successfully hold the location for five months, despite expecting to be replaced in 90 days. Over 100 of the unit were killed over this time, including Loomis and Parker. Kellin worked to understand the Dominion technology in the communications array in order to use it, though he had little success.

When Captain Benjamin Sisko and his crew arrived to help the unit, Ezri Dax helped Kellin with the array. Sisko ordered them to pause and switch to expose the subspace anti-personnel mines (known as "Houdinis") that had threatened them, and to turn them on the Jem'Hadar. In the process, Dax and Kellin became friends and he was impressed with the engineering knowledge she had.

Kellin was killed in the final Jem'Hadar attack against AR-558, while saving Ezri's life, before the arrival of reinforcements and relief from the USS Veracruz. (DS9: "The Siege of AR-558")

Kellin was played by Babylon 5 and Lost in Space star Bill Mumy. The actor had wanted to appear on Star Trek: Deep Space Nine ever since the show's first season, and was offered many alien roles. He waited until he was offered a Human role.
According to the Star Trek: Deep Space Nine Companion, Kellin's name is an homage to film actor Mike Kellin. Kellin appeared in such movies as Hell Is for Heroes, Midnight Express (in an Oscar-nominated performance as Brad Davis's father), and the original Sleepaway Camp (the last picture he completed before passing away). Mumy and Kellin worked together in one episode of Lost in Space; like Mumy, Kellin was a Twilight Zone veteran...having starred in Rod Serling's "The Thirty-Fathom Grave".