Princess Eilonwy (voiced by the late Susan Sheridan) is about 12 years old, and beautiful, designed with flowing blonde hair and a delicate figure. In the film, Eilonwy is portrayed as plucky, cynical, and talkative, as shown by her tendency to go off into rants at Taran for seemingly trivial reasons (though he can be snippy back in their mutual antagonism). Despite her naturally strong-willed personality, she can be affectionate (for she tries to get Taran's spirits up in one scene and seems fond of Gurgi) and emotional, as shown by her anxiety after escaping from the Horned King's castle and her genuine sadness at Taran's near-sacrifice during the film's climax; by the end of the movie, Eilonwy has dropped her defenses significantly and she and Taran begin to be romantically involved.
Eilonwy appears to be addled once she first appears, confused and thinking her reasons of being imprisoned in the Horned King's castle vague. Eilonwy appears to have magical capabilities, which is what saves both Taran and Fflewddur from the Horned King's prison.
Eilonwy joins the Eds' team the Atlantean Alliance in The Eds' Adventures of The Black Cauldron.