I utterly adore Nala. She's a strong character in my eyes, and yet I think she still retains a certain cub-like spunkiness, which she particularly displays during the "Can you feel the Love Tonight?" sequences. She's got dignity and yet she doesn't seem to take herself too seriously, I think - but what's really inspiring is the fact that throughout Scar's reign, though she is losing hope, she doesn't give up or back down. She stays true to herself, and tells Simba how it is and what needs to happen; but it's not just that, she motivates him. If not for Nala stumbling upon Simba in the first place, where would we be? She really does set in motion the events that are crucial to the survival of the Pridelanders.
And with that regarded, I think she's very vital to the plot and film. Plus, she's a really cute cubbeh, and she's gorgeous. I think Moira Kelly did a fantastic job as her voice, and I love how her voice is deeper but still has a feminine flair.
I love her.