Etro is a fal'Cie goddess in the Fabula Nova Crystallis: Final Fantasy series. She is a key character in the mythos, being the one who provided the material from which humans were made, and the one who protects the balance between the mortal world and the afterlife. Many of her actions shape the events of Final Fantasy XIII and its sequels, but her exact role in the other games is still unclear. Motomu Toriyama has stated that while Etro is the same deity across the games of the series, her roles in the universes of Final Fantasy XIII, Final Fantasy Type-0 and Final Fantasy XV vary greatly.
According to the official mythology, the universe of Fabula Nova Crystallis: Final Fantasy is divided in half: the visible world, or the world of the living, and the invisible world, or the world of the dead. The two worlds must remain in balance, or the universe might be destroyed. Etro was created by the god of light, Bhunivelze, who became the sole ruler of the living world by defeating his mother, Mwynn, and banishing her into the world of the dead. This did not satisfy him, as Bhunivelze believed all things were destined to die due to a curse laid on the world by his mother. To reach the realm of the dead and destroy her, Bhunivelze created the three deities: Pulse, Lindzei and Etro.
Etro was the second deity to be created after Pulse. The mythology at this point calls Etro and the others fal'Cie, even if all other sources know them as the gods and goddesses of Fabula Nova Crystallis: Final Fantasy lore. Bhunivelze regretted having created Etro, because he had unknowingly created her as the image of Mwynn. Bhunivelze feared Etro and gave her no powers to create anything of her own. Bhunivelze crystallized himself to sleep until his creations would find the door to reach the realm of the dead. While Pulse and Lindzei were busy creating the world and creating many more fal'Cie, Etro was lonely. It is revealed in Final Fantasy XIII-2 Fragments After, out of a desire for Bhunivelze's attention, Etro tore up her body and disappeared from the world while Lindzei used her spilled blood to create the first human: a girl named Yeul, who was made in Etro's likeness, and by extension, Mwynn.
Entering Valhalla, Etro found Mwynn as she was almost overtaken by the Chaos that festered as a result of Bhunivelze's attempt to reach the Unseen World. Before disappearing into the chaos, Mwynn gave Etro the task of protecting the world balance. The goddess grew fond of the beings crated from her blood and bestowed each of them with a piece of the chaos, which became known as their heart or soul. Etro also acted as their guide: when a person died, their soul would retreat to Valhalla and melt into the Chaos and Etro would retrieve that soul and guide it back to the Visible Realm to be reborn. Etro also created the laws within Valhalla that its resident creatures and Eidolons follow.
Final Fantasy XIII
Etro is mentioned in the Datalog entries, where she "sings a lament for the dead". Etro is revealed to have intervened during the War of Transgression 500 years previous by turning the l'Cie bestowed with the "burden of the beast", Oerba Yun Fang and Oerba Dia Vanille, into crystal before they could finish carrying out their Focus.
Though unseen in Final Fantasy XIII, in later installments it has been revealed that Etro aids Lightning by providing her, Snow Villiers, Hope Estheim, Sazh Katzroy, Fang, and Vanille with Eidolons, and releases Lightning, her sister Serah Farron, and their friends from the burden of being l'Cie after Fang and Vanille sacrifice themselves to create the crystal pillar to save Cocoon.
Final Fantasy XIII-2 Fragments After
Etro's act of mercy to Lightning and her group at the end of Final Fantasy XIII caused distortions in time as chaos emerges in Gran Pulse and drew Lightning to Valhalla. Though the goddess subdues the chaos, Etro is gravely weakened. When Lightning appears before her throne, Etro reveals she originally hated Lightning and her companions for the deaths they caused as l'Cie. Etro briefly links with Lightning to have her understand the isolation she has endured and her love towards mankind.
Etro reveals the countless deaths of many swirling within the chaos, along with the unintended suffering she caused for Yeul as the time distortion causes ripples in time that kill the seeress's incarnations while driving Caius over the edge. After giving Lightning her gift of sight to see various moments in time, Etro enters a deep sleep with Lightning choosing to stay in Valhalla as the goddess's champion to protect time and atone for her sins.
Final Fantasy XIII-2
As side stories reveal, Etro took pity on the deceased Yeul when her soul could not pass on like the others, and returned her to the Visible World in a cycle of reincarnation. Yeul gained the Eyes of Etro, which shorten her life to point of dying in her teens.
Due to her gift of seeing changes in the timeline, Yeul became the seeress of the city of Paddra. Etro gave her heart to Yeul's Guardian Caius Ballad to bestow him eternal life to protect every incarnation of Yeul. These acts, however, prove to be more of a curse than a blessing.
Caius begins his war on the goddess by altering time to suit the ideal conditions to bring the reality Etro has been preserving to an end. Caius has Noel Kreiss destroy the "Heart of Chaos", the source of Etro's life, so the chaos within Valhalla erupts from Etro's Gate and distorts the two worlds into a timeless realm where Yeul would no longer be endlessly reincarnated. In the secret ending Caius declares Etro dead and beyond any chance of revival.
Lightning Returns: Final Fantasy XIII
Because of Etro's death, no new people can be born as the souls of the dead lost their guide back to the Visible Realm. To uphold the balance between the new Visible and Unseen Realms, Bhunivelze intends to condition Lightning to replace Etro. Lightning rebels against him and destroys him after severing his hold on humanity's souls. After Bhunivelze has been felled, the incarnations of Yeul declare their intention to remain in Etro's place in the Unseen Realm. Caius allows the final Yeul to depart with Noel to the new world, while the other Yeuls remain behind as Nova Chrysalia is destroyed.
A cult named Children of Etro were a group of former citizens of the great city of Luxerion who defies the Order of Salvation, the very powerful religious society who worship Bhunivelze, and they consider Etro to have only lost her power. They believe that the Savior's task is equivalent to killing them and stage a series of murders in Luxerion with the victims matching the rough description of the Savior as a pink-haired woman. Etro is also depicted in murals in certain dungeons within the Dead Dunes telling the story of humanity's creation.
Final Fantasy Type-0
While Etro plays no major role in the story, she is mentioned in the Book of Void, a text chronicling the myths and legends of Orience. Prior to the final battle, Cid Aulstyne refers to Etro during his tirade over how powerless Class Zero is compared to him. It is also hinted that, through Rem Tokimiya, Etro kept watch over Machina Kunagiri during his efforts to rebuild Orience after the death of Cid Aulstyne and Class Zero and the waning of Orience's crystals.
Final Fantasy XV
Etro is mentioned by Stella Nox Fleuret at a treaty-signing party near the beginning of Final Fantasy XV when talking with Noctis Lucis Caelum about a legend from her home country of Tenebrae. The legend states that whenever Etro opens the Unseen Gate, the souls of the dead ascend as a streak of light towards the heavens and through the Gate. It is said that when those near death see the light, they are given power from the Kingdom of the Dead (although few people are able to see the light).
Stella states she can see the light, as can Noctis. Noctis guesses both of them have had a near-death experience though little else is currently known about the goddess or the power imbued on Stella and Noctis.