Eloi was a cross with deep blue eyes and partial Siamese features that welcomed Francis into the community of cats living underground after Francis was orphaned (despite the community had not a real leader, he was clearly an influential cat)
His actions are told by the same Francis in the Sixth book, Schandtat, and Eloi is described as an acculturated mentor for Francis but also as a fatherly figure for him.
Eloi is an important character in Francis's life, responsible to have given a cultural education of high level to Francis.
As the cats living underground with him and Francis, Eloi reads lots of books and was used to the plants grown inside the caves that were consumed with scent's inhalation as a soft drug.
(SPOILER): Eloi was the sole survivor (apart Francis) from the massacre of the community of cats they were living with. For years Francis believed Eloi to be one of the victims of the mass murder and the truth about his fate will be revealed only to Junior. Despite some shadows over him were lifted, there is no clear revelation about the ultimate fate of Eloi and Morlock (his son) told simply that Eloi was "murdered" before his birth.
Trivia: After he left the underground cave where the cats community was killed, he had mated with a she-cat. Later he seems to have been murdered and Morlock, his son, born by the she-cat, remained close to the underground cave (as a guardian of the mass-grave) to being close to the father he never seen.