So the merc is just told he’s to escort this girl across the mountains, through the forests, and get her safely to a kingdom so she can sign a peace treaty. He’s cynical and weary but he’s willing to make some money, so he’s on board. He gets to this village and meets his charge. She’s young, constantly looking at her feet, and speaks softly. She moves with a gentle shuffle and is very cautious at all times. So he thinks “okay, she’s a typical sheltered princess and is just being demure”.
Once out in the wild, though, she’s everywhere - disappearing all the time, collecting flowers or playing with animals. She tells him that she loves interacting with “the smallfolk” like himself. So now he’s really not having fun.
They get into a scrape - bandits, enemy army, whatever. He’s convinced she’s totally unprepared for this situation, but he’s going to give his life keeping her safe, because it’s his duty. Now he learns her secret: she’s a princess of the giants.
She’s got a magic dingus that shrinks her to human scale, but she can turn it off. Like when bad guys are attacking. She’s usually about 40 feet tall, but she grew up around humans. The quiet voice, the shuffling feet, the timidness around people? She’s grown up trying her hardest not to squash people one-eighth her size. The smallfolk.
So now there’s an enemy who got wind that the giants’ princess is on the way. What is the kingdom signing a treaty for? Why, to enlist the aid of the giants in an upcoming war. If the princess gets killed, the treaty is negated… So the mercenary and the princess have to get to the castle, avoid being spotted, avoid giving her presence away, and sign the treaty. Along the way, the mercenary needs to teach her how to live as a human-scale person. And she needs to teach him the lesson she’s spent a lifetime learning: how to be a good human being.