Basically the Hearts are diplomats, thinkers, social butterflies and the people who get terribly upset and protective of the commoners when someone threatens them. Despite being partially militaristic, they are not knights in shining armour. Socialising, they have discovered, is much more fun. In fact, they like parties very much, and don't make distinctions between factions. Morbid and sarcastic as they find the Moon Guard, they enjoy a relationship of friendly competition.
They are, most often, from families that have been in the Sun Guard for as long as history books have been bound, and while they have a few small branches (reminescent of knightly orders), they are mostly a colourful and diverse bunch with the same uniform. Some are loners a notch higher than freewarriors, but they still adhere to rank in a liberal manner. One cannot be ordered to join one of the branches - this is paramount. Asked to assist, yes.
All in all, in a world of humans where organisation is a show of culture, the Hearts show that the Caedaren can be organised, which actually is the truth... if in a slightly strange manner.