Jesaja is a gray Persian cat that appears in the Felidae movie and first novel.
After the discovery of Solitaire's body, Francis spot Jesaja and follow him underground until discovering the catacombs with hundreds of deceased's cats remains. Francis question the old cat, who claim to be "Guardian of the Deads", and realize that the death toll of victim was much higher than he expected.
Francis discovers how Jesaja play a fundamental (yet unoblivious) part for the "Felidae" project because he is the cat assigned to hide the bodies of the victims underground. He is unaware of the whole Felidae project and that he was in fact deceived and exploited to cover the murders, making him believe he was acting as a sort of a chosen one.
After the progressive debilitation of Claudandus, no more bodies were left for him to be guarded (they were instead left exposed to Francis' investigations). Jesaja attempted to discover why this happened and at that point was discovered and followed by Francis. Jesaja gives precious information to Francis, helping him to solve the case.
Trivia: His ultimate fate is described in the book. Francis and Bluebeard manage to convince him to come out from the catacomb and he is be adopted by an innkeeper.
Having spent most of his life alone and underground, it is unlikely he have children.
Despite his features in the movie, Jesaja is probably younger than Bluebeard and Felicity (or at least he has the same age) being born into the Laboratory of Doctor Preterius.