Well, first, Natsu looks a lot like Katsuki... Especially the messy hair, except Natsu's pink. And personality wise, he's kinda carefree and reckless in nature. He often brawls with the other members of Fairy Tail for... debatable reasons. But, despite all that, he is a fiercely loyal and protective friend. He is willing to go down fighting for his friends, regardless of how futile it might seem.

One could say Natsu has a straightforward mind, and often tackles issues with a "hands on" approach. And, well, his solution to problems often involve violence. But, at the same time, even when disrespected or faced with obvious hostility, Natsu rarely ever reciprocates the feelings, and often forgoes grudge-holding.

[Mavis seems very fond of Natsu but, then again, so is she of every member of her guild.]