That's true. I've noticed a few things like that. For example, Genis and Raine didn't really shine until they hit Tethe'alla, when they learned they were half-elves. Sheena didn't really shine until Volt, and so on and so forth.
For the most part, that's the case with both Kratos and Zelos. In fact, the game pretty much forces you to play at least two playthroughs in order to get the whole back story on everything, and for you to see both 'endings', even if there isn't too much of a difference, just no Zelos in one of 'em. I've played the game . . . countless times (5+ probably, orz), and sometimes I still get confused on where to go next, because of all the stories. That would have to be one good thing about the animation even if Genis is still being comic relief bait right now. hopefully that'll change when they meet Mithos. Same goes for Raine.
Nevertheless, Tales of Symphonia is still my favorite game. xD;
I know, right? ;_; They say Zelos' hair is hard to do, but let's face it, they had so much trouble with Kratos' they had to make it less awesome. :(