I just had a weird idea.
Purpose: Group needs someone to help guide them safely to a location.  Only one person knows the routes well enough.
Twist: However, he's depressed and drinking a lot.
Reason: He loves a girl and wants to marry her, but for some reason she won't marry him.
So I'm trying to figure out why she wouldn't marry him.  I thought, maybe she likes girls.  Wasn't a serious thought, but for some reason it stuck.
Was like 'hmmmm, so if she only likes girls, what would the guide do?'  Become a girl!  So guy needs special potion to turn him into girl.  Has group go find it.  They bring it back, he drinks it, turns into a girl, goes to the woman he likes, she's happy.  They promise to wed once he guides the group to their destination.