Mythra

Mythra is Rex' Blade in Xenoblade Chronicles 2 and Addam's Blade in Torna ~ The Golden Country.

Appearance and personality
Mythra is a blond haired Blade with an emerald Core Crystal on her chest and one of the Aegises. Her Core Crystal has a gap in it in the shape of an X after her and Pyra give half of their life force to Rex. She is often outwardly prickly when embarassed in contrast to Pyra, though she becomes somewhat more honest about her feelings through the course of Xenoblade Chronicles 2. Despite her brashness, she greatly cares about her friends. Mythra first appears in Torna ~ The Golden Country alongside Addam, her Driver. As Addam was the first person to awaken her, she inherited his fear of her power, similarly to how Malos, the other Aegis, inherited some of his traits from Amalthus. During the events of Torna ~ The Golden Country, Mythra is often made fun of for her inability to cook or being a 'simpleton,' but grows fond of her friends throughout her journey. Addam sees her like a daughter, and she also grows to see Milton like a little brother. However, it can be assumed she disliked her reputation and was perhaps envious of Jin, as Pyra is very different from her in these regards. At the end of Torna ~ The Golden Country, she appears emotionless, and later is completely grief stricken. Like Pyra, it is heavily implied that she falls in love with Rex over the course of Xenoblade Chronicles 2.

Torna ~ The Golden Country
In Torna ~ The Golden Country, Mythra first appears alongside Addam when they save Jin and Lora from one of Malos' Gargoyles. Her and Addam are also traveling with Milton, a young boy who joined them on their travels after his village was ravaged by a disaster. As the counterpart to Malos as an Aegis, her and Addam are attempting to defeat Malos. As Addam wants to fight Jin and Lora afterwards due to Jin being the Paragon of Torna, Mythra fights them alongside him. After the battle, Jin and Lora join their party, as they are also headed to Torigoth. After finding Torigoth to be razed to the ground by brigands, Mythra and the party fight Brighid, who initially assumed upon seeing the party that they were responsible for the destruction. This is cleared up when Brighid's Driver, and Addam's best friend,, halts the battle. Mythra later grows to respect Brighid as an equal in power and sparring partner. She also develops a sibling-like relationship with Milton.

Mythra, alongside the rest of the party in Torna, fight against Malos. In the final battle, Mythra calls upon Siren to fight Malos' own Siren. Upon seeing Malos' Siren attack Auresco, she unleashes her full power to the greatest extent she could. She defeats Malos and damages his Core Crystal. However, due to her fear of her power, she could not properly control her power, causing a large amount of destruction, sinking the Tornan Titan with an explosion that resulted in the death of Hugo. Her Foresight also goes into Overdrive, causing her to see visions of the far future, including her future meeting with Rex that would occur in 500 years. After she returns to the refugee ship of people who survived the sinking of Torna, she sees that Milton is dead, and Mikhail moves him away from her when she tries. In her grief, she bursts into flame, creating Pyra, and asks Addam to seal her beneath the Cloud Sea so her power could never be used again.

Xenoblade Chronicles 2
Mythra first appears in Xenoblade Chronicles 2 at the end of Chapter 3, when Rex's recklessness after 's death causes him to be in mortal peril. She would rather not have had to awaken at all, but did due to the desperation of the situation. Her circumstances are especially tragic in that Jin and Mikhail blame her for the death of Torna, though Jin also later blames Amalthus, as Amalthus was the one who killed the people of Torna. She meets Brighid again in Chapter 4, who contains a journal of her past lives and as such remembers to some extent her experiences with Mythra. She then switches with Pyra oftentimes throughout the story when needed. She also meets Haze, one of Lora's former Blades, again at the same time, who is now missing half of her Core Crystal and called Fan la Norne. Mythra dislikes what Haze's new identity is like, especially as Haze mentioned that she could not mention being the Blade of 'such a horrible man' in regards to Amalthus. It is also revealed that Mythra's Core Crystal was obtained along with Malos' when Amalthus climbed the World Tree. Pyra mentions that Mythra saw Addam as an honest, genuine, really good person, and that Rex reminds Mythra of him a lot. She is extremely shocked when Jin kills Haze.

In Chapter 6, she loses to Jin in a fight, who is able to outspeed her through the power granted to him by Malos. After this, her and Pyra are taken by Jin and Malos to the Cliffs of Morytha. At the end of Chapter 7, it is revealed that her and Pyra's goal to reach Elysium were motivated by their wish to ask the Architect what their purpose was before dying. However, being accepted by Rex regardless of the fact that her intentions in reaching Elysium would break her heart causes her and Pyra to awaken into their most powerful form, nicknamed by fans as Pneuma. At the start of Chapter 8, she asks Rex to choose a name to call Pneuma. She also temporarily allies with Jin so they can survive in the Land of Morytha.

In Chapter 9, she saves Jin from an attack from Amalthus. While she claims it was just because they were allied against Amalthus for the fight, it appears likely that she also just didn't want him to die in regards to the fact they were friends 500 years ago. She grieves for Jin after he dies from the strain he put on his body to defeat Amalthus.

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 the name of 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 sacrifices herself and 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

 * Mythra is an Aegis
 * Mythra fought Malos in the Aegis War 500 years before the events of 'Xenoblade Chronicles 2'.