When I am playing as Crono, I more or less feel like I AM Crono- I don't really consider this to be "creative license", but I suppose that is relative... However, once he is no longer a mandatory character, he becomes, for me, completely superfluous. I can take them out of the party to have more companions who speak, and yet I am still there.
Otherwise, it works as a concept, which is perhaps why Serge never ceases to be the lead figure in Chrono Cross. Therefore, I don't even find it reasonable to compare Crono to the other characters.
I don't think Ayla's language is the principal problem. None of the characters have an extremely sophisticated vocabulary. More expository and individualizing text could have been conceived for Ayla and subsequently grammatically destroyed- We might have taken it less seriously because of this, yet the content would be the same. Ayla is a very simple character. This simplicity is also part of her charm, yet, as heirs of our western culture which has thousands of years of history of science, art and philosophy (even if many of our contemporaries are not the most cultivated people, Chrono-fans seem to have some level of education and spiritual openness), we have trouble identifying with this level of simplicity. It also leaves little place for development as the amount of potential development seems to be relative to the existing complexity. By Ayla's terms, ANY amount of change in her character (the aforementioned contradiction as a possible example) represents a huge development.
To feed my Lucca argument, I will disagree that Ayla has little development with a few key points.
I've already stated my position that she grows a lot during the course of the game. She has to deal with a lot of issues, namely being chief of a tribe that is barely surviving near the beginning of an impending ice age, with a whole other species trying to kill them.
To supplement this, I'll quote your earlier post and bring my point full circle.
The only seeming development on the part of Ayla is in the form of a contradiction- She affirms that the law of nature is: Strong live, weak die. (no change law). She nonetheless extends a helping hand to the defeated Azala who, according to this law, has lost the right to live. At the very least this makes Ayla a normal human, contradictory and prone to compassion (or sentimentality if you prefer)
However, it is also often difficult to distinguish between character exposition and character development. Perhaps Ayla always had this contradictory tendency. We don't know.
We do know that the law of nature is also the law of her tribe: the strongest is the chief. If she had a contradictory tendency, she wouldn't be chief. What I mean is the tribe would likely see sympathizing with something "weak" as a character flaw in a younger Ayla (pre-chief status). If she had a consistent habit of letting the weak live and contradicting the tribe's seemingly
only law, I find it highly unlikely that she would have ever gained chief status. This leads to the most simple and logical explanation, her mercy/sympathy/pity toward Azala is actually character development. She's learning from the team and they're rubbing off on her.
With most of the characters, especially Frog, the development is more spread out. I posit that Ayla does not have any
less development, it's just more condensed. Her screen time is saturated with story.
All we know about Lucca is that she's nerdy, into science, is better at fixing things than building them, and thanks to the red gate, that she possibly blames herself for her mother's accident (though this has no effect on her actions or personality).