Myself, while they are both entirely pleasant to look at, it is not the image that drew me in. While playing the game for the first time, I was initially attracted to them because they were two of the more believable characters in the game in terms of motivation, dialogue and voice acting.
As I explored further, personality and backstory came into play. One of Lulu's traits that stand out strongly for me is her frankness. Think back to the scene in which she scolds Wakka for "replacing" Wakka with Tidus. Nobody else in the party would have done that. Her adaptability is also admirable. While her belief system is being revealed as fraudulent and crumbling before her eyes, she remains collected and simply deals with the new circumstances.
I like Auron primarily due to his selflessness. He's essentially sacrificed his life to serve others, from Yevon to Braska to Tidus to Yuna. Such devotion is far more heroic than his instrumental role in defeating Sin, in my opinion. Asceticism also appeals to me, so the fact that he was a monk kind of seals the deal. Of course, he also shares Lulu's startling bluntness. They bring the kids back down to Earth (er, Spira?), and I like that.
So how about you? What do you love about the characters? Or what don't you love about the characters? Let's talk Auron and Lulu.