Mythra

Mythra is one of the main characters from Xenoblade Chronicles 2. She is a Blade, which are personifications of weapons used by Drivers in the world of Alrest, more specifically an Aegis (or Legendary Blade) that can be wielded by the game's main protagonist, Rex. She created Pyra as a different personality of 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's voice can be heard saying, "Talk about a tight spot! Let me handle this!" 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 in-depth and announcing their official release date occurred on March 4th, 2021.

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.

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.

Trivia

 * 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. This modified design was added to Xenoblade Chronicles 2 in December 2018 as a downloadable costume, under the name "Massive Melee Mythra".