Ilia, a Deltan from the planet Delta IV, had been romantically involved with Willard Decker, who also later served aboard the Enterprise. She did, however, swear an oath of celibacy upon initiation of her service, a requirement for Deltans entering Starfleet.
Although Ilia was eliminated by a probe from the V'ger entity, V'ger created another probe that was an artificial near-duplicate of her to collect information and ease communications 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.
|Navigators of the starships Enterprise|
|USS Enterprise:||Arex • Bailey • Brent • Chekov • DePaul • DeSalle • DiFalco • Farrell • Hadley • Haines • Ilia • Kelso • Latimer • Leslie • Osborne • Painter • Riley • Stiles • Saavik • Tyler • Uhura|
|USS Enterprise (alternate reality):||Chekov • Darwin|
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 Willard Decker, was the inspiration for both the Betazoid character Deanna Troi and Troi's relationship with the Human William T. Riker. This also includes the Betazoids named after an Greek alphabet like the Deltans, the Betazoids' relatively very open attitude toward sexual activities, and Troi along with the rest of the Betazoids' ESP (Extra Sensory Perception) capability.
In the DVD commentary for the Director's Cut, director Robert Wise suggests 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 seems 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 is revealed that V'ger was modified by the source species of the Borg. It is 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.