Four of the vie main ones have quite a bit (Spike is lacking), Chomper has quite a bit, but, after that, I’d say, other than the parents, Ali is the next character who has the most development (other than perhaps Hyp & Co.)
Mo doesn’t really have much. Tippy doesn’t really have much. Peterano has some. Rinkus has a bit.
(Dil & Ichy have the most of any Sharptooth other than Chomper though.)
Shorty is sadly lacking in development (and wans’t even in the 14th movie!!!) Bron, well, we’d still like to know more about him.
Tria got some but we still have yet to know when she knew Cera’s father in the past and how.
Guido, well, he does learn to fly and stuff, but even he doesn’t seem to know his own past.
We don’t know too much about the Yellow Bellies (and some, not me, consider that a good thing).
Wild Arms we don’t know much about (and I’m not sure I care to), and Etta is a mystery.
We do know some about Mr. Thicknose but not much.
At least Legend of the Story Speaker showed us some of Grandpa Longneck’s past.
Ruby is also lacking in character development, save Star Day Celebration and Return to Hanging Rock where we seem to learn the most about her.