Even though she doesn't get a whole ton of screen time, Nala is The Lion King'sresident kung-fu expert (think back to the time when she and Simba wrestled, and she was all like, "Pinned ya! Pinned ya again!") and Simba's best childhood friend. She's just as tough as the boastful young prince, plus she can pin him to the ground whenever she wants.
Despite being stronger than Simba, she's still got mad respect for him: when they were escaping from the elephant graveyard together, he saved her from getting bitten by a hyena by swiping the hyena in the face.
When Nala runs into Simba in the jungle, she's shocked that he's still alive. But, she gets over her shock fast and reminds him of his responsibility to the throne. This is partly because Nala is very much a force of moral good and partly because of that mad respect we mentioned earlier.
When Simba finally hauls his butt back to Pride Rock, Nala is right there with him, ready to help him overthrow Scar. Although Nala was Simba's friend long before the two of them grew up—and once said, "Ewww," when informed by Zazu that she and Simba would marry—this does not prevent her from eventually having some romantic feelings for him.
When Nala discovers Simba in the jungle, the two realize that they're a little more than friends, which is an adult thing to do and a huge part of Simba's coming-of-age story: with Nala's help, he matures socially, emotionally, and romantically, coming into his own as king of the Pride Lands.
Nala finally ties the knot with Simba at the end. Which is good, 'cause we were shipping them really hard.