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 is different from other Deltans in only one area–as a girl, she had considerable contact with Humans because of her father's prominence as a Federation historian with special interest in parallels between civilizations of Earth and Delta. Something of a dreamer, Ilia eventually became fascinated with the "primitive" heroic qualities in Humans–this led to an interest in 23rd-century space exploration, in much the same way some Humans become intrigued with the heroic ocean voyages of the Polynesian people.
Since her own Deltan race had long ago lost interest in space voyaging (concentrating on their own inner-space), Ilia finally decided to join Starfleet herself and study at first hand the space adventuring in which the Human species were playing so prominent a role. As is true of the few other Deltans who have joined Starfleet, Ilia's natural job function was navigation--the highly evolved Deltan intelligence can handle the most complex spherical trigonometric complexities of space navigation as easily as a Human learns simple multiplication tables.
Ilia's past connection with Decker is a bit troubling to her. She met Decker while still little more than girl--her romantic "dreamer" nature saw him loom in her mind as a handsome, primitive young warrior, excitingly different from any Deltan man she knew. She then discovered that this "primitive young warrior" had unusual intelligence and charm. All this combined with Ilia's burgeoning interest in Earth voyages and voyageurs–her emotional Deltan zest catapulted her into love with this young Starfleet lieutenant. But Decker, already an experienced officer, knew of the dangers implicit in any such relationship–he also recognized that his own principal need was for the challenge and adventure he could find only in space as a starship commander some day. When Ilia's interest in him led to preliminary love-play, even the unconsummated sex experience left him so shaken, that he saw the trap in time. He fled, realizing he could not risk even a "good-bye"--another hour with Ilia might have brought him to a point of no return.
Ilia is now certain that Decker made the right decision–her affection for him was genuine, deep enough that she saw it would be selfish of her to interfere with his chance of one day achieving the ultimate freedom and challenge of starship command. Decker is equally certain he made the right decision about Ilia–and yet, strangely exotic and compelling memories of her still torment him at times.
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.