Best Friends and Fire Masters. Ace and Natsu have big little brother friendship(nakama). Natsu reminds Ace of his little brother Luffy only he’s less reckless than Luffy.
Natsu: Hey Ace, you okay buddy.
Natsu: you sure, you look so down.
Ace: Natsu.…you remind me a lot of my little brother…Luffy.
Natsu: Really? Let me ask you something. What is Luffy like?
Ace: Both of you like to eat tons of food .I don't think you can consider them eating I consider you guys swallowing food down without any gag reflexes and not being able to choke. Natsu like you showed me and taught me how to protect and care for the guild. always takes care and protect the Guild, just like how Luffy will fight for his Pirate Crew the straw hats and make sure they're safe.
Natsu: I can tell you miss him.
Ace: Yeah. I miss him and Sabo.
Natsu: The rest of the grand line has to know your gone for you stay safe. You’ve been given a second chance Ace. I need ya, Gray needs ya, Erza need ya, Lucy needs ya, Wendy needs ya, Happy and Carla need ya and the rest of the guild needs ya. We don’t care if you’re the son of an infamous pirate you’re our friend, our guild mate, our nakama.
Natsu: besides I wouldn’t have another fire Buddy spare with. Wanna go Fire Fist.
Ace: you’re on Salamander.