Well you know what...kinda was already thinking along those lines and to be honest thats how i came up with my guys looks. Your guy astride him and him asleep on the bed. :P 

I'm thinking your guy will attend the great festival being held in honour of his 21st but due to one thing or another (including a wig he's forced to wear in public of ashen hue that falls down to his breast pockets), he retires early, not quite feeling himself. He goes to bed and your guy slinks in, gets ready to attack but the prince wakes up and rolls him over, or somehow stops the blade and offers him something better. Not sure what that something better is yet entirely as this one's a bit of a mystery to me, but either books or knowledge in return for protecting him from other would be assassins?