Doctor Keridwen Llewellyn was a human scientist in the 23rd century, with an interest in magic and religion. Dr. Llewellyn was esper-blind, meaning that she had no telepathic capacity whatsoever. However, she appeared to be mildly empathic and had an uncanny knack for predicting the deaths of loved ones, discovered through her interest in tarot cards.
Around 2270, she travelled to the planet settled by the Fabrini priestess Natira and the people of Yonada, where she met Dr. Leonard McCoy. Upon the return of the USS Enterprise from its five-year mission, Dr. McCoy had gone to rekindle his romance with Natira, only to find that she had married following since their prior encounter in 2268. (TOS episode: "For the World is Hollow and I Have Touched the Sky")
McCoy joined Llewellyn on her scout ship, leaving the Fabrini colony. Their lectures on Fabrini culture eventually led them to Vulcan, where Llewellyn hoped to interview adepts of the Kolinahru and learn more of the ancient "mind-lords". Spock, a friend and former shipmate of Dr. McCoy, transported both doctors to the Kolinahru Monastery at Gol. Prior to that, Llewellyn had performed a tarot reading for Spock; among the cards that came up was Death, which unnerved her, given her accurate predictions of death in the past. While waiting for an opportunity to speak with the Kolinahru, Llewellyn sought solitude, fearing for the lives of her friend McCoy, and for Spock as well.
While at Gol, a renegade Kolinahru named Sekar - carrying the katra of the ancient High Master Zakal, a feared sorcerer of the Reformation period - hijacked Dr. Llewellyn's scout ship, with Dr. McCoy (seeking refuge from Vulcan's heat) onboard as an unexpected passenger. After eavesdropping on the Kolinahru gathered to discuss the matter, and once her esper-blindness was revealed, Llewellyn was chosen to accompany Spock and High Master T'Sai in pursuit of Zakal, which eventually led to the Romulan Neutral Zone; Sekar sought to bring Zakal's mind-powers to the Romulans ("the Declared", as he called them) in order to bring Vulcan back to greatness. Upon encountering the Romulan Bird-of-Prey that Zakal had commandeered, the injured T'Sai transferred the katra of Sotek, Zakal's former student who had later joined with Surak, to Dr. Llewellyn; her esper-blindness allowed Sotek's presence to go unnoticed by Zakal, culminating in the final confrontation between master and student on the bridge of the Bird-of-Prey.
At some point, the Bird-of-Prey's self-destruct had been activated to prevent Zakal from reaching Romulus and teaching his mind-powers to them. With their last acts, Llewellyn and Sotek sacrificed themselves to destroy Zakal, and save the lives of both Spock and McCoy. (TOS novel: The Lost Years)