The One
Behind the scenes
First appearance The Beginning
Last appearance The Beginning
"It was alien, not Yeerk. That was to be expected from the Yeerks, they were, after all, parasites, so you never saw the Yeerks themselves. But there was something very wrong with this particular alien. The face that filled the screen and more was a shifting image, a slow dissolve from what might be a robot's face, a machine with a rat-trap mouth and steel eyes, into a sweet, feminine, almost elfin visage, and last, and most enduring, into the face of Aximili-Esgarrouth-Isthill. [...] Jake stared back at the foul thing on the screen. I saw what he saw, and I felt as if my brain was shutting down. In that shifting alien face was every corruption, every evil, and such power that it seemed impossible it could be present in just the narrow confines of the onrushing Blade ship."

The One was encountered by most of the Animorphs in The Beginning. Shortly after the end of the Andalite-Yeerk War, The One encountered the Blade ship, allowing the remnants of the morph-capable Yeerk Empire to live so long as they became subservient to him. A few years later, The One attacked the Intrepid and kidnapped Aximili-Esgarrouth-Isthill, whom he assimilated. Ax's first officer Menderash-Postill-Fastill, who survived the assault, returned to Earth to inform Jake, who along with Menderash, recruited Tobias, Marco, Santorelli and Jeanne Gerard on an unofficially sanctioned mission into Kelbrid space, where they were found by The One. With no weapons to destroy The One or the Blade ship, Jake orders his crew to ram their ship, The Rachel, into the Blade ship.