Final Fantasy XIII Episode Zero
During one summer before going to attend a university in Eden to study history, Serah falls in love with Snow Villiers, whom her older sister Lightning doesn't approve of. While still being on vacation, Serah has a dream of her destroying Cocoon not realizing this was her Focus. Ignoring this, four days later she has been branded a Pulse l'Cie which is against the ideals of where she lives. Scared to tell Snow this, Serah breaks up with him as Snow demands to know why she's doing so and finally tells him the truth, that she has the Pulse l'Cie mark. Snow calms Serah down by reassuring her that after her Focus is complete she'll be returned back to life.
Two days later on the beach, Serah meets Oerba Dia Vanille whom opens up to her about problems about wanting to apologize to someone, which, ironically, at the time Serah was also dealing with. Feeling comfortable around Vanille, Serah tells her the dream she had of destroying Cocoon. Vanille breaks down apologizing to Serah revealing it's her fault that Serah was made a l'Cie, but Serah, not knowing of Vanille's involvement, does not understand.
The next night during a parade, Snow and Serah watch fireworks up in the sky on a vehicle discussing her Focus and what the outcome of it may be. Snow pops out an engagement necklace (which happens to be the logo of Final Fantasy XIII) and proposes to her. Serah being very excited and surprised accepts the proposal. After the firework parade, Snow and Serah decide to tell Lightning about the engagement and the Focus. Lightning being very angry believes Serah is lying about the Focus to use that for an excuse for her and Snow to run away together. To prove Lightning wrong, Snow and Serah venture out if they can remove the brand until they get invaded by PSICOM forces whom get Serah trapped in a silver liquid substance that pulls her into the Vestige.
Final Fantasy XIII
With Lightning finally coming to her senses realizing Serah was telling the truth decides to team up with Snow during the Purge to save Serah. However, by the time they reach Serah she is lying down on the floor weakened getting prepared to be crystallized Her last words to Lightning are to save Cocoon, she then closes her eyes and transforms into a tear-shaped crystal. Lightning thinks Serah is dead and that she won't come back while Snow being hopeful thinks the opposite. After the whole gang is turned into I'Cie themselves just like Serah was they wake up in a crystallized lake which Serah is a part of now called Lake Bresha. Throughout almost the whole game, Serah is in a crystal form until the gang completes their Focus by defeating Orphan. However, with every gain there is a great loss. This is applied to when Vanille and Oerba Yun Fang sacrifice themselves to keep Cocoon from crashing into Gran Pulse. The two turn into crystal while Serah returns into her regular form reuniting with Snow and Lightning. Serah forgives Lightning for not believing her and it's hinted that Lightning finally approves of Snow for Serah. Thus, a happy ending... until XIII-2 that is.
Final Fantasy XIII-2
Three years after the events of Final Fantasy XIII, Lightning is gone; presumed part of the crystal pillar with Fang and Vanille. However, Serah recalls events differently, and knows that Lightning wasn't crystalized despite everyone else believing the contrary. Snow has also disappeared; believing Serah's story, he chose to go in search of Lightning, but as of the game's start had yet to return. When a mysterious human from the future named Noel Kreiss, appears before her and says that he can take her to Lightning, who is in the future as a servant of the goddess Etro. Serah chooses to journey through time with Noel on a quest to be reunited with her sister.