Arslan is the prince of Pars, but is frequently misinterpreted by those around him as being a weakling, because of his effete appearance, naivete, compassion, and optimism. While these are true to a point, Arslan is also philosophical, intelligent, and charismatic, able to rally even the most apathetic of people to his cause. He is dedicated to his friends, especially Daryun, at one time threatening to kill an ally who had got Daryun into a particularly nasty situation.
It is later revealed that he is not of royal blood at all and, in fact, is a son of a foot soldier and a servant girl and simply was adopted by the Royal family
Whether by birth or by marriage, a man whose mother is a queen will find himself in the position of a prince. Hopefully, they end up as a prince charming and not a wicked prince who likes to burn peasant villages in their free time. Being a prince does not necessarily mean you are a nice person.