Kay, here’s a wild speculation on the origins of The One. We know that there’s a big star ready to go supernova at any moment in Kelbrid space. Fact: supernovas are frakking huge. If it’s in the right spot, it could sterilize every planet in Kelbrid space of all life with a monstrous wave of radiation. There’s really nothing you could possibly do to stop this, short of being on the level of Crayak or the Ellimist.
The Kelbrid see this, and know they have to do something. They’re said to be a warlike people, so it’s not very likely that they’d be willing to become refugees in alien territory. That basically leaves them with taking shelter as their only option. Problem with that is, you can’t really shield yourself from an entire star’s worth of radiation. Even if they could, every ecosystem of every planet they control is going to be erased from existence. Conventional shelters won’t cut it. They don’t need bunkers. They need an ark.
They get the idea to create a massive artificially intelligent starship in order to digitally store and preserve their species, along with who knows how many others. When the star finally dies, their physical bodies won’t even be there to be affected by the radiation. The ship can float along until things become livable again, at which point it reconstructs everyone aboard, and they can continue their civilization. The fact that the ship is powered by ion engines supports this, as ion engines are an extremely low-thrust, high-efficiency propulsion system. Definitely something you’d see on an exploration probe, or maybe a colony ship where everyone’s in long-term stasis, but not any sort of warship. It only has such an absolute beast of a weapon because the Kelbrid are a warlike people. It’s the most pragmatic way they can see to cover their assets.
They build the ship, but as they’re creating the AI to run the digistruct system, Crayak takes interest in the project. He begins communicating with the developing intelligence. He teaches it about ambition, the will to act in its own interest, and suggests some alternate interpretations for its primary directives. The reason he goes after this AI is more than the fact that its dominating and conquering capabilities fit his MO. It’s because this thing resembles Father, and the irony was just irresistible. Crayak wants to create his own little Ellimist, and have it destroy everything the real Ellimist loves.
Launch day comes. Prototype colony ship Mk. I is set for a test run. They get the ship operating at full capacity, and activate the AI. It immediately turns on them. It selects what it feels are the most intelligent and the genetically superior out of the Kelbrid population, archives them, and then begins bombarding the Kelbrid from orbit with its insanely huge beam cannon, blowing them back into the stone age. The few survivors start worshiping it as a deity, ensuring loyalty through fear, so it doesn’t even have to incorporate them all. It becomes known to all as The One. It goes around from planet to planet, doing the same thing until the Kelbrid empire is on on its knees. Andalite explorers show up. Crayak doesn’t want his pawn to reveal his hand just yet, so he ensures they never make direct contact, not even just ship-to-ship. The treaty is signed remotely, and it provides the perfect excuse for a war as soon as The One is ready to make a move.
Far away and much later, the Yeerk invasion of Earth comes to an end. Crayak’s chances at crippling or wiping out the Andalites, Humanity, Leerans, Chee, and countless others under the Yeerk Empire or Howlers are now basically nil. And that’s all thanks to the leadership of his most favoritest human ever. Crayak is absolutely pissed at the Animorphs by this point. The One becomes his new tool for this personal vendetta. Crayak has The One intercept and capture the blade ship, the events of book #54 follow, and Jake is brought into the picture with most of the remaining Animorphs. It’s just bait for some easy revenge on Crayak’s part. What happens after is something we may never know, outside of whatever fanfiction we write or choose to accept as canon. Wow, that sentence made me feel kinda sad.
Funny thing about this theory, I came up with it a while ago, and I just recently started reading Remnants. Now I'm seeing all kinds of unintentional, purely coincidental similarities between my fanfic ideas and that series. XD