Lenne is a tragic character, and not seeing or knowing much about her makes her somewhat mysterious throughout much of the game.
And she's not a "nobody". It's ultimately because of the fate her and Shuyin met that Yuna is continuing to look for more Spheres and the course of events plays out as it does for YRP.

FFX-2 isn't mainly about Lenne, but she still plays an important role in the story.