Luffy and Yn have a older brother and little sister relationship, They are polar opposite of each other Luffy who is optimistic, Goofy, and carefree, Yn is more mature, clever, and
Sassy, at first She thought luffy really annoying and nothing more than big joke to everyone around, and that he will never be king of the pirates,
But The more time she spend with them
She started to think highly of him,
Luffy deeply care about her and will do any thing to keep her safe, after learning that she is the daughter of doflamingo, he still care for her, and willing to keep her safe from him
Luffy can be bit overprotective when it comes to yn because he doesn't want to lose
Her like how he lost his brother Ace,