Being a Deltan Ilia has the ability to put images in others minds. She also has the ability to remove pain. Ilia met Willard Decker from Starfleet and the two had strong affections for each other. This was before the Deltans had oaths of celibacy when dealing with sexually inferior species. Decker left her home planet after he was reassigned and did not meet her again until she was stationed to the U.S.S. Enterprise as the new navigator.
On her first mission she was merged to the V'ger probe. This gave her even more abilities as she could walk through solid walls of the starship Enterprise as if it were made out of paper. Later, Willard Decker was absorbed as well. James Kirk listed the two as missing in action as they were absorbed and not killed.
Ilia was created for the second Star Trek series now known as Star Trek II. Persis Khambata was cast for the role. In the show she was going to wear a scull cap to give her a bald look. This series added the characters Ilia Willard Decker and a full Vulcan Xon to the regular cast with Spock being the exception. After the success of the Star Wars film the project was turned into a movie called Star Trek: The Motion Picture.
In the film Khambata shaved her head instead of wearing the scull cap and Xon was dropped for Spock who signed with the film. The actor who was to play Xon played a human commander of the Space Station that is destroyed at the beginning of the film. The film was an extended version of an episode for the show. The ending was changed however as Ilia and Decker were not joined to V'ger.