I think, at this point, it's obviously Cassie and Tobais. They aren't outwardly strong in any sense. Jake makes a point of having the Hork-Bajir follow him and Rachel 'cause the two of them are the fastest. Tobias is "totally out of shape" and Marco and Cassie "are too short to be fast." Marco, however, plays up the macho in his first sequence with the nether fjord, therefore making him weaker than Cassie.
And K.A. proves that they are the "weakest" (physically) by giving them something the others didn't have the guts to do: morph on their own. Cassie and Tobias are the first to morph without a push from someone else. Jake doesn't do it until Tobias does. And after Cassie morphs, everyone's willing to try it. But because Cassie and Tobias morph on their own, they are proven to be challenged. K.A. gives them an alternate destiny: that of a strong person on the inside.
So...there's something for you to think about. Talk amongst yourselves. ;o)