Mythra

Mythra is one of the main characters from Xenoblade Chronicles 2 and its prequel, Torna – The Golden Country. She is a Blade, a personification of a weapon (in her case, the legendary Aegis Sword) that can be wielded by protagonists Rex and Addam respectively. She is the Aegis' original form, having created Pyra out of shame as an alternate and less harsh facade for herself. Mythra uses a variety of light-based powers in combat, whereas Pyra uses flame-based powers. Both her and Pyra were released as a singular 2-in-1 downloadable fighter for Super Smash Bros. Ultimate.

Super Smash Bros. Ultimate
On February 17, 2021, during a Nintendo Direct, both Pyra and Mythra were announced as the fighters for Challenger Pack 9 for the Fighters Pass Vol. 2 in Super Smash Bros. Ultimate. They were released on March 4th, 2021. The reveal trailer begins with announcing Pyra, and during a battle with Link in which he throws a Remote Bomb at her, Mythra interjects from within Pyra, alerting her "sister" to the danger at hand, and offers to take over their shared body. Pyra then transforms into Mythra and continues fighting with Link, successfully reflecting the bomb back at him. A follow-up presentation showcasing Pyra and Mythra's moves occurred on March 4th, 2021, followed by their official release on the same day.

Mythra and Pyra function as one character, with players having the option to switch back and forth between them, similarly to how Zelda and Sheik functioned in Super Smash Bros. Melee and Super Smash Bros. Brawl. Mythra's moves include Lightning Buster, a slash attack that can be charged; Photon Edge, an attack that slashes opponents multiple times with a wide reach; Ray of Punishment, where Mythra slashes upwards and fires a beam of light downwards; Chroma Dust, which is similar to Ray of Punishment but features multiple downward shots; and Foresight, a dodging maneuver similar to Bayonetta's Bat Within that leaves opponents open for a counterattack. Mythra's Final Smash is Sacred Arrow, in which Rex appears and the two rain multiple beams of light on opponents.

When compared to Pyra, Mythra is a much faster fighter overall, walking faster than Little Mac, and being a faster runner than Fox. However, Mythra's higher overall mobility comes at the cost of being less powerful on offense, as most of her attacks lack the knockback strength to KO fighters at realistic damage percentages. Mythra is also lighter than Pyra, as she shares her weight with the Ice Climbers, Lucario, Wolf, Villager, and Steve.

Prior to being revealed as fighters, both Mythra and Pyra were Legend-class support spirits. Mythra's spirit battle involves fighting female Corrin, representing Mythra, and Shulk, representing Rex, on the Battlefield form of Gaur Plain. When equipped, Mythra's spirit gives the fighter a chance of landing a random powerful attack.

Classic Mode route
Mythra shares the same Classic Mode route as Pyra, which pits them against pairs of fighters who are similar to each other, and both Master Hand and Crazy Hand appear in the final round regardless of the current intensity.

Palutena's Guidance
Mythra and Pyra share this stock conversation with all other DLC characters except Piranha Plant. It was originally used for Mewtwo, Lucas, Roy, Ryu, Cloud, Corrin, and Bayonetta in Super Smash Bros. for Wii U, who are downloadable fighters in that game.
 * Pit: Who is THAT?
 * Palutena: I have no data on this fighter. I can't believe it!
 * Viridi: It must be an intruder from another dimension!
 * Pit: Whoever it is, the goal remains the same: to fight and win!

Quotes

 * "Prepare yourself."
 * "Think you can take me?!"
 * "Nuh-uh."
 * "Guess again!"
 * "Foresight!"
 * "Concentrate, Rex."
 * "Sacred Arrow!"
 * "...Seriously?"
 * "Ray of Punishment!"
 * "Chroma Dust!"
 * "Swap!"
 * "All yours!"
 * "Pyra!"
 * "Let me at 'em!"
 * "You're done!"
 * "Lightning Buster!"
 * "Photon Edge!"
 * "*giggle* Called it."
 * "Peak form."
 * "[with Pyra] This... is the power of the [without Pyra] Aegis!"

Trivia

 * As discussed in the Mr. Sakurai Presents "Pyra & Mythra" showcase, both Mythra's in-game model and spirit have seen changes from her original design in Xenoblade Chronicles 2, adding clothing to cover up her cleavage and legs to meet the CERO's "A" age rating. This modified design was added to Xenoblade Chronicles 2 in December 2018 as a downloadable costume, under the name "Massive Melee Mythra".
 * Mythra's down taunt quote, "Think you can take me?", and her swap-in line, "You're done!" are references to the battle quotes of the Ardainian Soldier enemies in Xenoblade Chronicles 2; the former was patched out in an early version of said game.