Lieutenant Ilia was a female Deltan Starfleet officer in the 23rd century. She was in the navigation and helm branches of the Command division. In the mid-2270s, she served aboard the USS Enterprise under the command of Captain James T. Kirk.
Years before her assignment, she had been romantically involved with Will Decker on her home planet Delta IV. She would meet him again on the Enterprise. She did, however, swear an oath of celibacy upon initiation of her service, a requirement for Deltans entering Starfleet.
Although Ilia was killed by the V'ger probe, V'ger created another probe that was an artificial near-duplicate of her to collect information and act as a liaison between V'ger and the "carbon units" that "infested" the Enterprise. The probe mentioned that the Ilia unit was "no longer functioning". The duplicated Ilia probe was similar to Ilia down to the molecular structure, as a medical exam later revealed. The probe included both her memories and personality, though both suppressed by V'ger's programming.
Kirk later listed both Ilia and Will Decker as missing instead of killed, following the involvement of both in the conclusion of the crisis. (Star Trek: The Motion Picture)
Ilia was played by Persis Khambatta. The character was originally proposed for Star Trek: Phase II. The show's production materials described the character's abilities and background – notably, that she was very intelligent, an "esper" and that almost everything with her people was sex-oriented. Khambatta had been cast in the role for the series, and had done costume and makeup tests which were used for the motion picture. (The Making of Star Trek The Motion Picture)
Ilia, as well as her relationship with Will Decker, was the inspiration for both the Betazoid character Deanna Troi and Troi's relationship with the Human William T. Riker. This includes the Betazoids, like the Deltans, being named after a character in the Greek alphabet, their relatively very open attitude toward sexual activities, and their ESP (Extra Sensory Perception) capability (manifesting in Troi primarily as empathy).
In the DVD commentary for the Director's Cut, director Robert Wise suggested Ilia was a last-minute replacement for the unnamed transporter accident victim who was killed earlier with Commander Sonak. Decker's surprised reaction at her arrival seemed to support this.
Ilia can also be seen in the first two story arcs of the Star Trek newspaper comic strip.
In The Return, by William Shatner, it was revealed that V'ger was modified by the source species of the Borg. It was suggested that Ilia and Decker were fused to the Borg Collective, in five-dimensional space, by V'ger and that Ilia might be the Borg Queen seen later.
According to the video game Star Trek: Starship Creator, Ilia's father is named Serilli and her mother is named Cherlas. She has a brother named Senn and a sister named Anya.
Her mirror universe counterpart was depicted as being a crewmember aboard the ISS Enterprise in The Sorrows of Empire. She was likewise the lover of Willard Decker. She was killed by Spock using the Tantalus field in 2272.