Roy sitting on salad
Roy was a holographic fly, designed to test micro-surveillance spy techniques for Starfleet Intelligence.
In the 2370s, Starfleet commissioned Doctor Lewis Zimmerman, the leading expert on holotechnology, to create a tiny spy hologram for the "Trojan Horse Project". Roy, being small enough that no one would notice, was successful. He was then given back to Jupiter Station, where Dr. Zimmerman kept him as a pet.
Haley was always telling Roy to stay off the food, especially Doctor Zimmerman's salad. In 2376, when the Doctor came to the Alpha Quadrant to treat Doctor Zimmerman for a fatal disease, he got so annoyed that he smashed a book on Roy. (VOY: "Life Line")
- Although this may seem like the end of Roy, it is unlikely that physical trauma could destroy the holographic insect, and his program was presumably re-initialized later on.