It's textbook, really. Or, more accurately, parchment, because you've either found yourself or always been in a time (it doesn't necessarily have to be medieval) where swords - and possibly sorcery - are the weapons and the mindset of the day; at the very least, the rough and tumble attitude that comes with a knife's edge permeates the land. And you know how to wield a weapon, be it a blade, your own body, or something else entirely, so you've put it to good use.
You've become a sworn sword to protect someone.
It could be out of the goodness of your heart, out of heritage, or stemmed from a desire for compensation or power. No matter what the cause, however, one fact remains the same, and that's how you cannot let emotions cloud your judgement.
Of course, that may be a little easier said than done. When you're so close to a person, in order to keep them safe, and when you put your very life on the line...
...ah, but you must persevere. Your duty does not allow you to falter.