Pyra

Pyra is Rex' Blade in Xenoblade Chronicles 2. Like Malos, she is an Aegis, a legendary and especially powerful Blade.

Appearance and personality
Pyra is a woman who wears a red outfit and has an emerald Core Crystal on her chest. After she gives Rex half of her life force, this Core Crystal has a missing area in the shape of an ‘X’ on it. Pyra’s personality is likely based on what Mythra felt she lacked or aspired to be during Torna ~ The Golden Country, and is also quite similar to Jin during the events of Torna as such. She and Mythra call the Architect "father". Unlike Mythra, who is more brash and puts up a more prickly front, Pyra is outwardly kind, like Jin. Additionally, she is skilled at cooking, another trait she has in common with Jin and differs from Mythra in. Pyra can be assumed to have a somewhat dark sense of humor, as shown when she responds “''I could never do that! I don’t have a knife''” when Nia mentions that Pyra could eventually backstab Rex.

Pyra, like Mythra, is immensely guilty about the events of the Aegis War. They both want to stop Malos and reach Elysium in order to ask the Architect why he made them and die. This changes over the course of Xenoblade Chronicles 2, where they grow to love Rex and the world. After being accepted by Rex despite their betrayal of him through their intentions in reaching Elysium, Pyra and Mythra transform into Pneuma.

Torna ~ The Golden Country
Pyra is a form of the Aegis, a powerful Blade which was one of two Aegises that fought during the Aegis War 500 years before the events of Xenoblade Chronicles 2. She is a weaker form created by Mythra so Mythra could seal herself away after Mythra was overcome with grief following the destruction of three Titans in the Aegis War. After the Aegis War’s end, Pyra was sealed away on Addam’s ship, which was then sank deep beneath the Cloud Sea.

Xenoblade Chronicles 2
Approximately 500 years after the events of Torna ~ The Golden Country, Pyra is discovered sealed within the Ancient Ship by a crew of elite salvagers. The young Rex was first to find her, and touched the core crystal on the sword in front of her, prompting Jin to kill him. Rex then woke in a grassy field with Pyra, who explained this was a memory of Elysium, and she wishes to go back there. But because Rex is dead, she chooses to spend half her life force to revive him, thus becoming his Blade, which is symbolized by an X shaped Core Crystal appearing on his chest, in exchange for Rex promising to take her to the real Elysium. Pyra then helps Rex fight against Jin and Malos, and with the help of Nia, Dromarch, and Azurda, the two of them manage to escape, but crash on Gormott after the battle. Later on at Gormott, Pyra learns that she and Rex share wounds from battle due to the fact they share a life force.

In Chapter 3, when Iona is kidnapped, Pyra goes missing, as she headed to confront Akhos and Malos after Malos ordered her to come alone in a dream and see what they want for Iona’s safety. Rex and his party show up to fight Jin and Akhos, with the resulting battle resulting in ’s death. After Rex recklessly continues to fight due to his grief over Vandham’s death, Pyra gives way for Mythra to awaken in order to ensure Rex’s survival. Pyra proceeds to then switch with Mythra as needed throughout the game.

Later on, Brighid talks to Pyra in secret about how Blades should be honest with their Drivers, seemingly because she has an idea of what Pyra and Mythra's true intentions in reaching Elysium may be.

In Chapter 6, Pyra is taken by Malos, though she ensures the survival of her friends by threatening to destroy herself with Siren unless Malos and Jin left them alone.

It is revealed in Chapter 7 that Pyra and Mythra originally wanted to reach Elysium to ask what their purpose was and finally be allowed to die. However, Rex still accepts them and fights Jin and Malos with resolve for them. His acceptance of them causes them to awaken into Pneuma.

In Chapter 10, when Rex is tested by the Architect through a vision, Pyra requests the Architect to stop it after seeing Rex suffer throughout what he experienced. It is revealed that Pneuma was one of the three cores in the Trinity Processor which was used to maintain the Conduit and later used by The Architect to collect data from the Core Crystals of Blades in his attempt to restore humanity, making Pyra and Mythra both Blades awakened from Pneuma's Core Crystal. After Pneuma transfers her remaining life force to Rex’s Core Crystal, the Aegis’ Core Crystal falls off of his chest, allowing him to awaken Pyra and Mythra again, who are now in separate bodies.

Lore

 * Pyra is a weaker form created by Mythra, which is supposedly the reason that her power is fire compared to Mythra's light.
 * Addam sealed her beneath the Cloud Sea, and she remained sealed as such for 500 years.
 * Pyra and Mythra are both forms of the Trinity Processor Core Pneuma.