Comportment is the way a person behaves — not only what they do, but how they carry themselves: how they walk into a room, how they speak, whether they seem careful, proud, or rude. It is a rather formal, old-fashioned word. You meet it in school reports, etiquette writing, and talk about manners; a teacher might praise a student's calm comportment. Younger people almost never say it in chat. They would say behaviour, manners, or “the way you act.” Though actually it sits a little closer to bearing than to behaviour in the wide sense: it often carries a quiet judgment, good or bad, proper or not. The matching verb is comport yourself, meaning to behave in a certain way, usually a proper one.