Depictions of some of The Ones forms.
(Spoiler Alert.)
The One is an entity far more powerful then a standard life form, but somewhat below Ellimist or Crayak. He is the last member of a race called the Eondero. The Eondero were at war with a race called theCaladon. He was a scientist working on a top secret project called thecollection device. It purpose was to allow Eondero soldiers to literally absorb enemy Caladon soldiers, their thoughts, experiance, essance, and physical attrabutes.
The One had other plans however, he planned to use the device for himself. He started absorbing the other scientists working on the project. The facility was quickly empty of all but the One. When a patrol ship investigated, the One played innocent. He lured them into docking, then he absorbed them. He continued this pattern. He attacked and absorbed both Caladons and Eondero, drunk with the power he was gaining. Both races formed an alliance to fight him, but by that time he had absorbed thousands, and possessed all kinds of powers.
It was a tough fight lasting decades, but the One defeated them both. The One continued his absorbtion of sentients, but one day he realized that absorbing normal, every day people was having minimal effects on increasing his power. He found that only by absorbing special, important people, like Aximili, could he add anything to himself, so he decided to increase his power the old fasion way, by conquest. When he found the Kelbrid, he saw them as an excellent subject race. He used his advanced powers and technology to make them serve him. The Kelbrid are individuals, but they are all linked telepathicly, a hive mind. The One found that when he sent them into battle, their violent, negative emotions, and even fear, fed him. It fed him as though that group of battleing Kelbrid was one being, because of the telepathic link. It was not much, but it added up. The emotional energy feeds on has been building up slowly over time. This is one of the reasons he desires conflict.
The One has had dealings with both Ellimist and Crayak. The One is powerful enough to keep ether from just wiping him off the map instantly with no consequences, but the rules set up by Ellimist and Crayak keep him from effecting anything on a grand, cosmic level without bringing Crayak or Ellimist down on him like a hammer. In a fight with any of them he would lose. He is below them. He is quite jealous of their power, and like Crayak he seeks to beter understand the fabric of space time and become more powerful. He is working towards those goals. The game between Ellimist and Crayak had kept eather from noticeing the One's deeds, and The One keeps a low profile.
Drode and the One have also had dealings, and even fought each other. Drode and the One are on an equal level in terms of power.
Part of The One was on the Bladeship. He had heard of the Animorphs search for Ax, and he wanted to lure them in with the Bladeship so that he could absorb the famed Animorphs. He hired the crew as mercenaries and used it as bait. He never expected Jake to order them to ram the Bladeship.