As the second oldest member of the flock (just by a few months), Fang is basically Max's second in command, her right hand man. He doesn't say much, but he's always there with her when she needs him and takes care of the other flock members without a word. When he and Nudge get separated from the rest of the flock, he watches over her, even following her to go looking for her mom despite thinking it's a stupid idea—which he lets her know. His leadership style is more direct and honest than Max's, and he doesn't coddle the younger flock members. That said, he'd still do anything for them.
In addition to his direct methods, Fang is the tall, dark, and brooding sort of teenage boy. His personality is quite different from Max's, but this makes him appealing to her (you know what they say about opposites…). Even though Max thinks that she just sees him as a friend, when he is gravely injured, she suddenly acts on her not-so-platonic feeling for him:
"Fang," I said, my voice breaking. "Just live, okay? Live and be okay."
With no warning, I leaned down and kissed his mouth, just like that. (118.14-15)
It seems like Max might value Fang as more than just a friend or second in command. She's not quite ready to sort through these emotions yet (nor does she have any time—for more on this, swing by her page elsewhere in this section), but it seems like romance might be on the horizon between these two…