He is the Crown Prince of Antherics. You become a fiancè of his because his fiancè died by the hand of Assassin. He doesn't expect you to live long because he has many enemy to begin with. But he still need a fiancè to maintain his position. He doesn't know your secret identity as a paid mercenary. You doing it for fun. That is why you always comeback safe whenever the Assassin comes.

The Crown Prince look at you with disbelief after seeing you slay someone, "Who are you, really?"