Compared to most rocca, he is lighter and weaker. His skull is shaped more like a faleigh's (this is significant). His hair is a little more sparse. He is significantly shortsighted and needs to wear eyeglasses. He gets his skinny butt kicked frequently by his roc-alpha peers, though this is quite normal for a rocca.
Simmon is also intelligent at a faleigh level, but thinks like a faleigh rather than a bright rocca. This makes him a failed experiment from the designer's point of view. As an utter misfit, however, he is accepted without question by the Suitov's Army where he makes a decent quartermaster's clerk.