I'm honestly not sure where this puts me - I think probably more on the introvert side of things, as I can't imagine feeling re-energized by being in a large group of people - but on the other hand, I do always enjoy being in a group that I feel part of. So I suppose it's not that I seek solitude, but more that it is always there on either side of time with others. Maybe this is because of my only-child status - the default was being on my own. I can imagine that for a child from a bigger family, the default would be a group setting, whether you liked it or not.
Anyway, I'm coming to the conclusion - now, for the moment - that contrary to expectations, Dash is my extrovert and Mabel is my introvert. If you know either of them in person, at least the second of these statements might come as a surprise.
Dash, even though he has gone through stages of crippling shyness (see four-and-a-half, for instance), really loves being in a group setting once he feels comfortable there. I think that's why he's enjoying school so much this year - he knows he belongs there, and it's his rightful space among others. He has always hated being alone - sending him to his room would create more tantrums than there were to begin with - and he is still incapable of playing on his own for any length of time. He craves company.
His sister, in contrast, has always seemed like the outgoing one, but she's also the one who can entertain herself quite happily. She really seems to need time to immerse herself, several times a day, in play with the dollhouse people and the dinosaurs and the elephants and the dogs and the Strawberry Shortcakes with all their intricate family dynamics and crazy goings-on. I think it's how she processes everything that happens. (I know I said she's going to be a lawyer, but she'll be a writer too. I'd put money on it.) And at school, her challenge is often to play (nicely) with others instead of just doing her own thing.
Of course, their personalities aren't set in stone, now or perhaps ever. And I think intro/extroversion is more of a scale than an either/or. But it's fascinating to see these little pointers emerge, and to wonder what use, if any, I can make of this pop-psychologizing.