Relationships: Serah
"You can't run away from yourself. Serah tried to tell me, but I wouldn't listen."
Serah is not a playable character, is rarely seen in the game (even then, it's mostly flashbacks), and spends most of the time in a crystal coma. Yet she is undoubtedly the driving force of the story. She is the undying source of motivation for Snow, the haunting memory that causes Lightning to re-evaluate herself, and the catalyst that shapes Vanille's attitude. Let's look at the relationship between Lightning and the most important person in her life - her baby sister, Serah.
A Reason to Persevere
"My parents died. I have to be strong for Serah, so I thought I needed to forget my past. And I became Lightning."
Serah seems to define Lightning's self-perceived role in life. Lightning and Serah became orphans in their teens. It placed a great burden on Lightning, for she felt she had to change drastically to be about to strong enough to deal with it. Lightning is supposedly a persona born to support Serah. Oddly enough, Serah is actually supporting Lightning, because being Serah's protector was the concept that supported Lightning by distracting her from her grief and fear. When Serah is taken by the Pulse fal'cie, Lightning doesn't hesitate to give up everything to get onto the purge train. She also angrily states that the fal'cie took away her life. Whether she really does have no other bonds aside from Serah, or Serah is just the only one that matters, these actions imply that Serah is her main focus in life.
Projection and Idealization
Despite their close and caring relationship, Lightning doesn't really understand Serah that well. When Serah told her about being chosen as a l'cie, Lightning didn't even believe the person closest to her. Because Lightning had acted as Serah's guardian for so long, she fell into the parental mistake of seeing Serah only as a child, not recognizing that her baby sister had grown up.
"You became a l'Cie, now you're gonna marry this idiot?
And you think I'm gonna buy that. Full points for originality."
Lightning also idealizes and projects her own feelings onto Serah. She blames herself for Serah's engagement to Snow. Supposedly, if she had spent more time with Serah, Serah wouldn't have been lonely and would not have ended up dating Snow. Unlikely. Serah is the one leaving Lightning. It is actually Lightning who is lonely. However, to avoid looking weak, she projects these feelings of loneliness onto Serah to reinforce their relationship as guardian and dependent.
Recognizing her Fortitude
It is only when you take a step back, can you see the big picture. Once she was separated from Serah, Lightning had more time and a better perspective to reflect on her relationship with her sister.
"What happened to Serah- All I could think about was, 'What could I have done?' I hated myself for not trusting her. It hurt too much. I couldn't face it."
Light clearly regretted how she had ignored Serah's news regarding being chosen and getting engaged to Snow. That started the process to understanding that Serah has grown up, and learning to let her go. When she acknowledged that Snow was a good person, she started acknowledging that Serah may be a better judge of character than she thought.
Lastly, the most important thing Light may have learned in regards to her sisterly relationship is that a person can be close to you, even when the distance is great. At the vestige, Light was quick to believe that Serah was dead, while Snow insisted on a more optimistic interpretation of being crystallized. If Serah wasn't in her sight, in her grasp, she was gone. It was the same overprotective mentality that made her hate the idea of letting Serah date Snow. By the time the party got to Gran Pulse though, Light was talking to Serah from afar, finally realizing how powerful and necessary faith can be. She learned what Serah knew at the very beginning: you have to believe in those that you care about.