Harry has all the puzzle pieces that will eventually come together to make him who he is. But at ten, he's still young enough that he spends most of his time being corrected and yelled at and and told he's underfoot, so he doesn't have the confidence (yet) to be as-- Harryish-- as he eventually will be. At ten, he still stutters and so avoids speaking. He's two/three years away from his first battle, which I think is a moment that changes a lot of things for him.

Viola is quite different! Her father won't die for another three years, so she's entering into about the last time in her life when she doesn't have some kind of concern about how to secure her future and get by floating around in the back of her mind. Her and her brother are about as inseparable as they'll ever be, I think, as they're just on the cusp of their social training sharply diverging.

Marius is shy, serious, and has no friends. Oh, wait.

The backstory I've made up for Segundus has him being raised by an uncle who was an Oxford don, so I think he also didn't spend vast amounts of time in the company of other children, or at least only the children who shared the relatively unusual social world of the university. So, still shy, still a nerd, probably slightly less self-conscious, if only because he hasn't yet begun the social decline that makes him feel like he's floating on the fringes of a social group he doesn't really belong to.