Ilia was a Deltan navigator aboard the Enterprise who was taken by V'Ger and replaced by a probe in her image in Star Trek: The Motion Picture.
Ilia was a lieutenant from Delta IV who joined the Enterprise as navigator after its refit. As a Deltan, she belonged to a species known for a strong sensual nature, and she had taken an oath of celibacy while serving in Starfleet. She had previously been romantically involved with Willard Decker when he was stationed on her world.
When the Enterprise met V'Ger, the entity scanned the bridge and abducted Ilia. It then sent back a mechanical probe built in her form to observe the ship and its carbon-based crew. Decker tried to awaken the memories of the real Ilia in the probe and used that link to learn more about V'Ger. At the climax of the film, the Ilia probe joined Decker as he merged with V'Ger, and the two vanished together with the machine.
Ilia's connection with Decker gave a human core to the story of the first Star Trek film, and her probe gave V'Ger a voice.