Theory

Theory is a Rare Blade in Xenoblade Chronicles 2. She is initially the Blade of Waldemar, though later can become the Blade of Rex, Nia, Mòrag, or Zeke. She is often referred to as Praxis's sister. Theory is an ice elemental Blade who uses the Chroma Katana weapon type and is classified as a Tank. She can be obtained from Theory's Core Crystal.

Appearance and personality
Theory is a female Blade with blue eyes and hair. Her outfit is predominantly a deep blue with bronze ornamentation. Her hair is tied back into a bun with a bronze ornament. Ice-like spikes jut out from her head, waist, arms, and chest. Her left eye is covered by an eye patch with a smaller ice-like spike coming out of it. Her Core Crystal has has a V-like shape and is identical to Praxis's.

Crystal Clear
The quest Crystal Clear becomes available upon purchasing Core Crystal Hunters from an informant shop in the Empire of Mor Ardain or Garfont Village in Uraya, the party learns of a bandit group targeting those who are transporting Core Crystals to Torigoth. While the party is looking into the rumor in Torigoth, Praxis and Theory jump the party in an attempt to rob them. During the ensuing fight, Ardainian soldiers rush to investigate the commotion, forcing Praxis and Theory to retreat. Mòrag explains the situation Iannus, a high ranking soldier at the Torigoth Relay Base, and manages to get his cooperation in organizing an ambush to stop the Core Crystal Hunters. The party then heads to Valdorm's Mound, where they ambush Theory. However it is revealed the Core Crystal Hunters knew they were coming all along. The party then fights Theory's driver, Waldemar, and Praxis. During the fight, Ardainian soldiers wipe out a bandit support squad, killing Praxis' driver. Praxis returns to her Core Crystal. Waldemar tells Theory the two of them are leaving Praxis behind.

Blade-Sharp Memory
Once Praxis has been reawakened and has reached the fourth level of her, the party can begin the quest Blade-Sharp Memory by visiting the Royal Guard Barracks in Fonsa Myma from Chapter 7 onward. Theory throws a lance with a note attached that nearly hits Nia. The note instructs the party to look for a Thunder Crystal in the Old Quarry in Uraya, and then stand by Garagorm's Arch in Gormott. Once the party reaches Garagorm's Arch, Theory throws another spear and runs away before the party can recognize her. The second spear directs the party to exterminate specific titans. Upon completion of the side quests Pest Control 1, Pest Control 2, and Pest Control 3, the party makes their way to Nharil Central Plaza in Mor Ardain where Theory confronts the Praxis about her past. Theory tells Praxis to read the final message before running away. The final note tells the party to head for the Lost Capital of Judicium. Before leaving the party can choose to tell Praxis about her past as a member of the Core Crystal Hunters. Upon arriving Theory reveals that the locations on the spears all pertained to places she and Praxis had been before. Theory, wanting only to be with Praxis again, fights alongside Waldemar and two other Drivers in order to kill Praxis's driver and get her back. After the battle Waldemar commands Theory to kill Praxis, to which Theory refuses. When Waldemar attempts to finish Praxis himself, Theory kills him. By killing her own driver, Theory is forced to return to her own Core Crystal, loosing all her memories in the process. In her final moments she tells Praxis she never wanted to fight her and laments how she will forget Praxis.

Rough Diamond
After Theory is reawakened by a new Driver, the "Rough Diamond" between Praxis, Praxis's, driver and Theory can be viewed in the Orl Cloudway: Fonsett in the Leftherian Archipelago. Theory, who no longer remembers Praxis acts distant towards her. Praxis tells Theory that she promised the old Theory that she wouldn't for get her, and that the old Theory wanted the two of them to be friends. Praxis's driver tells the two of them that they are essentially sisters, an idea Praxis likes though Theory has reservations about.

Theory and Praxis
Once Theory has been reawakened and the player has seen the Rough Diamond Heart-to-heart, the Blade Quest Theory and Praxis can be started by visting the in Fonsa Myma from Chapter 8 onward. Pandoria calls Theory a "Cold Fish" and asks Theory if she could smile every so often. Theory tries, but Pandoria is unimpressed. Praxis then spots two Urayan children, Veero and Vaio, crying. Praxis wants to see what the cause is, while Theory doesn't see that as necessary. When Praxis approaches the kids they recognize her and run away in fear. When the party catches up with them again, the kids explain that Praxis and Theory had attacked their father a year prior. Praxis attempts to explain that they are no longer the same people they were before, but Theory cuts her off stating how it'd be for the best if the two of them left. The party convinces the two of them to not let their past define them and to reconcile with those they've hurt. When Veero and Vaio take them to their father, Volette, three drivers attack them before the Praxis or Theory can explain the situation to him. Following the battle Praxis and Theory attempt to think of ways to prove that they've changed, however their banter alone is proof enough. Volette says he's willing to help them apologize to the people of Fonsa Myma. He suggests that they make Whelzaman Cookies for the upcoming Harvest Festival. After baking 50 Whelzaman Cookies, Praxis and Theory hand out the cookies to the local children. Seeing the children happy causes Theory to smile. Nia tells Theory she ought to go and celebrate, but Theory states how all of this is because of Praxis. Rex and Mòrag explain how it is because of the both of them that the townsfolk are smiling. When Praxis calls Theory over to her, Theory sarcastically remarks how "her sister gets lonely without her". Though Praxis is initially defensive, upon realizing Theory called her "sister" she lights up.

Sisterly Love
After completing the Blade Quest "Theory and Praxis" the Heart-to-heart "Sisterly Love" between Theory, Theory's Driver, and Praxis can be viewed at the Lyta Oasis in Gormott. When Praxis rushes ahead, confides in her driver that she wishes Praxis would calm down a bit. Despite her concern, Theory affirms that she cares for Praxis and her driver. She thanks her driver for reuniting the two of them before returning to Praxis.

As an enemy

 * Mediator Theory
 * Plunderous Theory

Gallery
Theory