Yeah I'm hazy on The One as well. But from what I can remember and this handy quote from the Animorph's wiki, it was just an alien on the same overpowered level as The Ellimist.

"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."