- “I'm Princess Eilonwy. And you're in bad trouble, aren't you?”
- ―Princess Eilonwy
Eilonwy is portrayed as brave, 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 for being imprisoned in the Horned King's castle vague.
Princess Eilonwy is a 12-year-old girl with a delicate figure, fair skin, pink lips, blue eyes, and long, flowing blonde hair that goes past her hips. She wears a dark violet band on the back of her hair, a pink skirt with a light pink underskirt, a white petticoat, a dark violet bodice, long off-white pink puffy sleeves, and black flats.
Eilonwy appears to have magical capabilities, which save her, Taran, and Fflewddur from the Horned King's prison.
In the film, Princess Eilonwy is taken hostage by a wicked emperor known as the Horned King, who hopes that her magic bauble could find the legendary Black Cauldron. She and the handsome farm boy Taran first meet in the lower ruins under the Horned King's castle (once the home of King Rhitta) when the addled princess is searching around and attempting to find a route to escape.
Instead of leaving, Eilonwy allows Taran to come with her and escape from the Castle. There, the two come across the mystic sword, Dyrnwyn (and Eilonwy expresses vague disgust at Taran merely taking a sword from a dead body) and free Fflewddur Fflam from his imprisonment. The group of friends later travels to the realm of the Fair Folk (thanks to Gurgi, who didn't lie to Taran and the gang) and reunites with the pig Hen Wen. The king of the Fair Folk sends Doli with them as an escort to the Marshes of Morva to retrieve the Cauldron.
After they receive the Cauldron by trading away Dyrnwyn (which Eilonwy briefly protests against), they are captured again by the Horned King who then unleashes the Cauldron Born. But Gurgi sneaks into the castle and rescue his friends without the Horned King noticing anything. Taran prepares to sacrifice himself, shocking Eilonwy, who insists he reconsiders, though Taran claims he has already made his decision, without her elaboration on it. After Gurgi takes over and sacrifices himself into the Cauldron, the friends trade the Cauldron back to the Witches of Morva in exchange for Gurgi back. The bargain is granted, and, thanks to Gurgi, Eilonwy and Taran end up kissing each other.
During the release of The Black Cauldron, Taran and Eilonwy were walk-around characters at the parks. So far, they have never returned to the parks. They also appeared on murals within The Walt Disney Story attraction and the Gurgi's Munchies and Crunchies restaurant.
- Eilonwy is a major character in The Chronicles of Prydain book series by Lloyd Alexander, upon which the film was loosely based.
- Eilonwy is not included in the official Disney Princess line-up or in any of the merchandise. This is most likely due to Disney's general tendency to disassociate themselves from their movie due to its insuccess and much darker tones (maybe also because of her preteen age unlike the other Disney Princesses), despite the cult following it gained on home video releases. Plus, the cuts and the re-editing and reworking the film has gone through after its completion, even in part of the soundtrack, would make it an "incomplete" movie.
- Princess Eilonwy has been nicknamed by fans as the "Forgotten Disney Princess" because of her little-known status and the fact she is not an "official" Disney Princess, despite being a princess by birth, like Kida Nedakh from Atlantis: The Lost Empire.
- In the novels of The Chronicles of Prydain, Eilonwy had a habit of running around barefoot. Pre-production artwork for The Black Cauldron shows that Disney's version of Eilonwy was to be barefoot as well.
- In The Chronicles of Prydain, Eilonwy's bauble does not float, nor does it change its color from anything other than the golden light it gives. It is also unbeknownst to Eilonwy herself but in the series, her bauble's real title is The Golden Pelydryn.
- Another distinction from the movie and original series is that Eilonwy is a descendant from the Welsh mythological god, Llyr.
- The character of Eilonwy in the books is described as having red-gold hair, but she is a blonde in the film.
- Unlike most Disney Princesses and heroines, both Eilonwy and Kida possess a magical item instead of animal sidekicks.
- Hayley Mills was originally cast for the role of Princess Eilonwy and got to record some scenes, but for reasons unknown, she was replaced by Susan Sheridan.
- In some products, the color of her dress is depicted as blue.
- Eilonwy's name was coined by the author Lloyd Alexander based on Welsh names with elemental meanings such as white brow or elm. Eilon means "fawn" in Welsh, while the suffix -wy means "water".
- Although she was revealed to be taken by the Horned King prior to her meeting Taran and her first appearance in the movie, Eilowny's kidnapping alone was never seen on-screen.
- Eilonwy's parents are never seen. The novels state that her parents are already dead. This would make her an orphan - just like Taran. Also, this would technically make her a queen, not a princess, unless she is not ready to take the throne and has someone to fill in for her duties until she is of proper age or has an older sibling.