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Species Blade
Gender Female
Japanese VA Ayako Kawasumi
English VA Sarah Borges
Appearances Xenoblade Chronicles 2
This article is about Theory in general. For gameplay information, see Theory/Gameplay.

Theory (セオリ) 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[edit]

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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[edit]

After finding some information about Core Crystal Hunters, the party learns of a bandit group targeting those who are transporting Core Crystals to Torigoth. While passing by Obra Street 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 to 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 predicted their plan, Waldemar, Theory's Driver, and Praxis arriving behind the party for a pincer attack. The party then fights Waldemar and Praxis. Waldemar is disatisfied by the direction the battle is going, gauding Praxis and Theory to fight harder. Meanwhile, elsewhere, Ardainian soldiers wipe out the bandit's support squad, killing Praxis' Driver. Praxis returns to her Core Crystal, leading Waldemar to retreat despite Theory's protest at leaving Praxis behind.

Blade-Sharp Memory[edit]

While the party is walking by the Royal Guard Barracks in Fonsa Myma, 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 enemies on different Titans, then go to Nharil Central Plaza. Upon completion of the three tasks, Pest Control 1, Pest Control 2, and Pest Control 3, the party makes their way to Nharil Central Plaza in Mor Ardain where Theory throws another spear with instructions. Frustrated at this, Praxis tells whoever threw the spear to reveal themselves, leading to Theory confronting her. The party recognizes her as Praxis's former companion while Praxis questions her about the messages she has been sending them. Theory understands that her plan failed, telling Praxis to read the final message before running away.

Theory's final note tells the party to head for the Lost Capital of Judicium. Upon arriving, Theory reveals that she had been trying to jog Praxis's memory, by throwing the spears she loved and sending her to places she and Praxis had been before. Theory, wanting only to be with Praxis again, vows to kill Praxis's Driver to get her back. She begins the battle by throwing icicles toward the party, intercepted by Praxis. A battle ensues, during which she fights alongside Waldemar and two other Drivers. After his lackeys' defeat, Waldemar begins to panic while Praxis reasons with Theory. Her Driver commands Theory to kill Praxis, to which Theory refuses. When Waldemar attempts to kill Praxis himself, Theory kills him. By killing her own driver, Theory is forced to return to her own Core Crystal, losing 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 her.

Rough Diamond[edit]

After Theory is reawakened by a new Driver, Praxis asks Theory to come with her to Orl Cloudway, though the latter is reluctant to do so. The Heart-to-Heart Rough Diamond between Praxis, Praxis's, driver and Theory takes place south of the Fonsett Waters' entrance into Orld Cloudway in the Leftherian Archipelago. Theory, who no longer remembers Praxis, acts distant towards her. Praxis tells her that she promised the old Theory that she wouldn't forget her, and that her former self wanted the two of them to be friends. Praxis's Driver can either ask Theory to consider the other Blade's feelings or notice that the two are alike. In both cases, 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[edit]

After Theory and Praxis's trip inside Orl Cloudway during Rough Diamond, the party visits Fonsa Myma's Commercial District. While there, 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 Theory 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, and the kids accept lead to the way. When Veero and Vaio take them to their father, Volette, he recognizes them and, after fearing for his children's lives, calls three mercenary Drivers to attack them before Praxis can explain the situation to him, not helped by Theory's own attempt. 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, solving a few of their problems at the same time. 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[edit]

The Heart-to-Heart Sisterly Love between Theory, her Driver, and Praxis takes place while they are visiting Lyta Oasis in Gormott. When Praxis rushes ahead, Theory confides in her Driver that she wishes Praxis would calm down a bit, though she loves spending time with her. Her Driver can remark one of two things.

Her Driver notices that the both of them behave like real sisters. Theory admits that she isn't so sure herself, though she does worry about Praxis's carelessness. Her Driver can also point out that the both of them understand each other now. Once again, Theory is unsure, though she recalls Praxis telling her that neither of them remembering each other meant they had a "clean slate for the future". Theory affirms that she cares for Praxis and her Driver and thanks them for reuniting the two of them.

Praxis finally realizes that Theory and her Driver stayed behind, calling them over. Theory returns to Praxis, telling her Driver to come with them too.

As a party member[edit]

Main article: Theory/Gameplay (XC2)

As an enemy[edit]

As an NPC[edit]

Theory can be found at a random location in Garfont Village after Rex takes over the Merc Group, provided she is not equipped to her Driver or sent on a Merc Mission. The specific locations she may be found at are listed here.


I would be interested in going along too...I suppose.


  • Praxis and Theory's awakening cutscenes follow the same choreography, albeit mirrored from each other. Both Blades begin by popping a sphere made of their respective element, then slide in a half circle back to their starting point. Both then lift their arms up before summoning their Blade weapons and slashing around.

In other languages[edit]

Language Name Meaning
United Kingdom flag.svg English Theory
Japan flag.svg Japanese セオリ
France flag.svg French Theory
Germany flag.svg German Theorie
Spain flag.svg Spanish Theory
Italy flag.svg Italian Theory
China flag.svg Chinese (simplified) 星织
Taiwan flag.svg Chinese (traditional) 星織
South Korea flag.svg Korean 세오리