Natsu and Gray where in the fairy tail guild and fighting like usual like a cat and dog, Gray uses ice magic which was the opposite of Natsu with his fire magic so it made since they don't get along to well, Grey has a laid back personality and only gets serious when it calls for it

both of them have a friendly rivalry but protect each other when needed it

Natsu: "Put your damn clothes on! Your in your boxers again Gray!"

Gray: "I can't control it when I'm fighting! They just come off"