If Earth is a chessboard, then the animorphs are definitely Kings as much as they are Knights. Taking any single one of them out, means ending the game for Earth. In a normal chess match, this means that if you put any of the kings in checkmate, they are all killed. The death need not be physical; it could be a metaphorical death, as in Book 31, marco says clearly, that if Tom took Jake's father, Jake would take Tom. Regardless of whether Tom won or Jake won, Jake loses either way, since he would reveal his identity as an animorph, lose his father and brother (traumatizing him) and following that, break down. Subsequently, the animorphs would each lose it; Jake was the center, and if he fell, they would all follow.
The Yeerks each are a chess piece; most of them are Pawns, the Sub Vissers are mostly rooks; powerful but limited to straight directions, and the Vissers are Queens; The most powerful on the planet. The Council of Thirteen are all Kings, though they are different from the animorphs in the sense that they do not participate in battle, and are never met except via hologram. They have no knights, which is why they often cannot trap the animorphs. (Any decent chess player will know that the knight is the third most valuable piece after the King and Queen, as it can move in nearly any direction and are often used to trap pieces) Tom is the only exception, as he is a pawn, and after Temrash's death, his second yeerk eventually reaches the end of the board (discovers the morphing cube and escapes with it) subsequently being promoted to a Queen (strongest piece in the game).
Humans would be pawns that belong to neither black and white; the Yeerks infest humans, adding those pawns into their numbers.
Since this game is a little bit different than conventional chess, and there are far more pieces than a conventional chess board, these allow for multiple kings, queens, rooks, knights, bishops and so on. However the King status is the same; taking them out will end the game for that board. Also, a single piece may not be only limited to one role; The Animorphs are an example of six knights, as well as six kings. A single loss to any one of them will compromise them all, therefore they each have to maneuver well and carefully.
FYI, But the Andalites are often thought to be Queens, because of their power, but they also Kings; taking them out will end the game.
Ultimately, the Ellimist doesn't interfere too much (Because every move he makes allows Crayak a counter move, and Crayak's move are all about destruction, which is far easier to do than to create; this means that the more moves he allows himself, the more moves he allows crayak, and the effects of crayaks moves will have a further reach than his own)
The only 'direct' interference he has ever made would be to 'save the Hork Bajir' (an umbrella term which allows him to interfere enough to save the hork bajir, ie. returning Tobias to the same location, creating the valley, leading Jara and Ket to escape on their own, giving Tobias his morphing power back) this eventually turns out to be a move that helps ends the war, considering how the Hork Bajir were eseential for Jake and the Animorphs to establish control over the Pool Ship. We are never told whether Crayak allowed this (though it would be implausible for him not to notice, or stop it, therefore implying that he simply made another move in exchange and left it to his own pieces (The Yeerks) to undo the Ellimist's attempts)
And technically against Crayak, he didn't move Earth physically at all. He pulled the strands of space and time, effectively shifting the earth forward or backwards through time to dodge it; minimal interference, maximal effect. He has always played by this way; making the most change with minimal interference.
