Kymble was a character, based on Harry Kim, in The Doctor's fictional holonovel entitled Photons Be Free.
Kymble was a Trill hypochondriac who served aboard the USS Vortex. Along with Tulak, he arrested the EMH character with the intent of reprogamming his holographic matrix at the orders of Captain Jenkins. He was very worried that they might damage the EMH's medical database, and there would be no one to help him if he caught an alien virus.
Unlike the real Kim, Kymble wore his hair straightly slicked back, which prominently showed the Trill spots on his forehead. He was attacked by Three of Eight in an attempt to help the EMH escape in "Chapter 7: The Escape". (VOY: "Author, Author")
It was never stated, either onscreen or in the script, whether Kymble was a joined Trill. Like Harry Kim, Kymble was portrayed by Garrett Wang.
From the Star Trek Encyclopedia (4th ed., vol. 1, p. 449), "Photons Be Free was presumably published some time after the conclusion of the episode, albeit in revised form. We suspect that the final version of the character was less like Harry, and may even have had a different name."