Ilia

Lieutenant Ilia was navigator on the USS Enterprise (NCC-1701) during the V'Ger incident of the early 2270s.

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.

During a probing from the V'Ger entity, she relieved the burns from Pavel Chekov's hand before Christine Chapel tended him.

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.

When Spock entered V'Ger's interior wearing a thruster suit, he saw a representation of Ilia floating in the expanse and attempted a mind meld.

Admiral James T. Kirk later listed both Ilia and Will as missing instead of killed, following the involvement of both in the conclusion of the crisis. 

Background information
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.

Apocrypha
According to Gene Roddenberry's novelization of Star Trek: The Motion Picture, Ilia's rank was lieutenant junior grade.

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.