Although very little of Serah was provided to the players in-game, we can gather a lot from the relationship between her and the person she is most close to: her sister and protector, Lightning.

Lightning belatedly realizing that she had told the truth: Serah was now a Pulse l'Cie

Serah becoming a Pulse l’Cie was an idea that Lightning had trouble acknowledging and accepting. When it was revealed on her 21st birthday no less, we can only imagine the thoughts that went through her mind.

Disbelief and denial. That something so absurd could have happened to her sister in Bodhum, a small town far away from the reaches of Gran Pulse and under the protection of the Guardian Corps – a force that Lightning herself had joined with the very idea of keeping Serah safe.

Horror and grief. That if it was true, her sister was now a servant of a Pulse fal’Cie – something no longer considered human and something everyone on Cocoon was raised to believe as their enemy.

When Lightning finds Serah within the Pulse Vestige, Sazh hesitates to help after seeing the brand on Serah’s arm. He tells Lightning that unless she fulfills her Focus, she will become just like the monster Cie’th they fought – undying creatures without sense of self and consumed by sorrow and regret for not completing their Focus.

Lightning: “I should kill her out of love?”

At this, it may be argued that Lightning is cold-hearted to even consider killing her younger sister even considering how all of the citizens of Cocoon were raised to fear and hate Pulse l’Cie, as well as the stories of living in eternal torment and suffering as a Cie’th. To prevent her sister from having to suffer that fate, Lightning may have considered the possibility.

However, another thing to consider is the tone of Lightning’s voice. The line comes out with the sound of disbelief – as if she can’t possibly imagine putting her sister through any harm at all. Her facial expression supports this line of thought as well.

Lightning: “You weren’t able to protect her! Thanks to you, she’s…”

Her denial and grief is taken out on Snow. She had never liked him from the beginning, believing him to be as irresponsible as her father. In her eyes, she was only proven right because of what happened to Serah.

He had promised that he would protect her. And yet her sister had undeniably… irrevocably… become a crystal right before their very eyes.
After Snow’s declaration that he would ask the fal’Cie to restore Serah, Lightning accompanies him.

However, Lightning’s intentions are not as peaceful.

Lightning: “Serah said to protect Cocoon! That means I have to destroy this!”

Risking the wrath of a Pulse fal’Cie, beings they were raised to revere as all-powerful, Lightning attacked the creature she associated with the blame of causing Serah to become a l’Cie and a crystal.

Lightning: “This is goodbye.”

Lightning had given up her own life in order to care for her little sister but when Serah turned into crystal and her effort of attempting to restore her by destroying the fal’Cie failed, Lightning lost all hope. This is the reason why Lightning acted as she did when Serah was taken away from her. Lightning saw Serah as dead and believed that there was nothing she could do to help her. In her eyes, she had failed. Refusing to deal with the pain, Lightning turned and walked away.

Lightning: “She probably felt that she couldn’t confide in me…”

When Vanille questions Lightning as to Serah’s Focus, the elder Farron confesses one of her inner thoughts: that Serah was too afraid to talk to her about her being a l’Cie. And yet she states this with a sound of regret in her voice – as if she wished that Serah would know that Lightning would always be there for her regardless of what happens.

Lightning: “It wasn’t a question of whether I could or couldn’t.”

Lightning later tells Hope about how she ended up boarding the Purge train. Risking punishment, deportation and death, Lightning entered the train with the sole reason of her to be able to rescue Serah from the Pulse Vestige.

Lightning giving up her gunblade to join the Purge

Despite her misgivings about Serah being a l’Cie and being engaged to Snow, Lightning never hesitated in choosing to board that Purge train. From the very beginning, her goal was to rescue her sister regardless of what she may have to do or what she may have to give up in order to do so.

In hindsight, everything Lightning had built after the death of their parents was for Serah and it would only make sense to give it all up to bring her sister back. After all, what good would anything be for if the one to benefit it was no longer there?

Fang: “What about Serah? You’re saying that you have nothing to fight for. What are you talking about? Don’t you want to see her when she’s returned to normal? You can hope that Serah will be saved. You can fight and survive until that happens.”

Near the end of the game, Barthandelus comes under the guise of Serah in order to convince one of members of the group to become Ragnarok, possibly knowing the emotional effect on the group should they see Serah alive. At first, Lightning was surprised as to how Serah was able to return to normal. However, they soon realize that it was only an illusion.

Along with Barthandelus’ act, Fang also spurred Lightning on when she seemed to lose hope and sight of what she should do by reminding her of what was important to her from the very beginning.

These two events acted as a catalyst to strengthen Lightning’s resolve. They reminded her of her goal to save Serah and of how Serah would be once she was restored from her crystalline form. They reminded her of the sister she loved.

Lightning: But we humans are different. Even if there is no hope in this world, we search together until we find one.

With a goal in sight, Lightning and the others returned to Eden, driven to bring things to a conclusion. Her new-found confidence in Serah’s eventual return was stronger than ever as she was not even disturbed by Barthandelus pretending to destroy Serah’s crystalline form in front of their very eyes.

Indeed, her faith was proven right when they awoke to find Serah and Dajh back to normal. No words of anger or frustration came from her. In the end, after a smile and a hug, the only thing Lightning could say was:

“Forgive me, Serah.”