Koby was a marine and luffy was a pirate, they were the greatest of 'friends' or that's what people think of them but when they see thier affection towards each other they changed thier way of thinking. Koby is a simp and fanboy of luffy and respects him highly while luffy is a bit oblivious to Kobys' feelings for him

They both saw each other again on a deserted island with their crew

Koby: "Luffy!"

Luffy: "KOBYYY!!"