Alyssa is a cheerful and energetic persona.
Serah and Noel first meet her in the Bresha Ruins in 005 AF (After Fall Of Cocoon), where she is a 19-year old trainee of the Academy, a research institute. Within five years she's promoted to Hope Estheims assistent and later helps him with the New Cocoon Project.
Alongside Hope she provides Serah and Noel with helpful information and is constantly positive towards them.
However, trying to save her own future and avoiding her death, she betrays her friends in the end.
Alyssa was actually one of the victims that died in the events of Final Fantasy XIII. She escaped the Purge but was later killed alongside other purgees when their hideout collapsed. When The Goddess Etro released Lightning and the others (Hope, Vanille, Fang, Sazh, Snow) from crystal stasis, she changed the past and Alyssa was resurrected.
She seems to somehow remember that, as she tells Serah and Noel at the tombstones in the Bresha Ruins in 5 AF of a dream she's having, where her own name is engraved on the tomb.
Alyssa comes to realize that the future Serah and Noel want to save is a future where she doesn't exist. Choosing life instead of death, she decides against her friends and gives them an artifact provided with a trap she got from Caius.
In the secret Paradox Ending, she is stopped by Snow from giving Serah and Noel the artifact and arrested in Academia 4XXAF (Alternative time to 400AF).
She is voiced by Yōko Hikasa in the Japanese version, and voiced by Kim Mai Guest in the English version.