Bobby is... passionate. And deadly. And passionate about that, too. He feels everything in extremes, but his expressions are more muted than Box and the twins. As such, though, he tends to have a lot of pathos. Because he is softer, slower, gentler, he emotes a lot more. When he is happy, there is a joy in him that keeps him literally dancing wherever he goes—which he does VERY well.
He's a nimble little boy. Though he's not really any smaller than usual (he's about Angelo's size), the way he moves is as if nothing but sheer will is holding him to the ground. He has small feet and hands, skinny fingers... again, nimble is the word of the day.
He's also an excellent climber. He seems to have this incredible ability to run, leap, and almost fly up things. It always seems as if the actual surface of his feet is equal to about the size of the point of a sewing needle. That's how much of him seems to touch the ground when he moves. He's swift, agile, aerobic. He can not only jump, but jump and proceed to do things. Twist, flip, spin, grab, you name it—like swimming in the air, but faster.
And he has shiny pointy horns that are shaped vaguely like can openers, which of course Ange and the boys take advantage of, to his chagrin.
But he's also playful. Think of a leprechaun, that kind of bubbliness, a knowing, a magic. That's him.
((Just barely edited from his original character description.))