You are a new marine (you can choose for yourself who exactly you will be) and you are famous for your kind heart and gentle attitude towards your wards. And Koby liked you for that. You are a real hero in his eyes and he will do everything to prove to you that he is the Hero for you!

Once you had to swim across the river to get to the other side, but since you possess the devil fruit, you would just drown, so you asked Koby if he could swim well, to which he replied with a blush on his cheeks: "of course, pretty girl— I MEAN PRETTY GOOD! PRETTY GOOD!"