First introduced in Super Smash Bros. Melee, Zelda and Sheik were one of very few examples of "stance" characters in Smash. They're also indicative of a specific trend that's occurred with players and stance characters in the Super Smash Bros. competitive scene. While other fighting games have characters designed as "stance" characters specifically, Smash handles stance changes as completely different characters. Competitive players have often found that one stance/form in particular is more tournament viable than the other in all scenarios of play. Melee's Sheik was clearly better than Zelda, and the same may be true for Pyra/Mythra in Super Smash Bros. Ultimate.
While other characters like Persona 5's Joker or Final Fantasy 7's Sephiroth have form changes, they're not the same as examples like Pokemon Trainer or, in this case Pyra and Mythra. Joker and Sephiroth gain Arsene/One-Winged Angel only after certain criteria, whether that's damage received or damage dealt, among other nominal factors. Examples like Pyra and Mythra simply switch with down-special, but it seems one of these characters is showing as clearly superior to the other. In terms of competitive play, Super Smash Bros. Ultimate's latest DLC character may emulate how Zelda/Sheik was in Super Smash Bros. Melee, which isn't necessarily a bad thing.
Pyra/Mythra Almost Emulates Melee's Zelda/Sheik
In casual play, which is obviously the majority of playtime spent in Super Smash Bros., players may have never noticed just how much better Sheik was compared to Zelda. Conceptually, the two characters are supposed to complement each other in terms of capability. Zelda would be a strong zoner-style character, while Sheik could build damage up close using combos. Some may not have even know players could swap between Zelda and Sheik at the time, but competitive players are absolutely well aware of that capability. There's a reason why players select Zelda and hold down the A button when matches start: Sheik is infinitely better than Zelda in competitive play.
Very early on players learned that, despite being significantly faster than Zelda, Sheik was also just as powerful. In some cases, aerial attacks like forward air in particular, Sheik was notably stronger than Zelda. While the competitive meta had to shake out a bit for characters like Falco and Captain Falcon, Sheik ended up higher in the meta early like Marth. Fast and strong frame data, chain-grabs and guaranteed follow-ups on nearly all characters, and overall an incredibly strong presence in neutral. Players would be hard-pressed to find any "Zelda-only" competitor in Super Smash Bros. Melee tournament play, and the same could potentially happen for Mythra in Ultimate.
Mythra Is Already The Better Choice In High-Level Play
In a similar scenario, Pyra and Mythra are Super Smash Bros. Ultimate's newest "stance" characters, but Mythra is clearly the standout choice. Super Smash Bros. Ultimate's overall balance is more even across the board, so it's not necessarily as disparate as Zelda/Sheik, but Pyra/Mythra aren't exactly a ying-yang relationship. Conceptually speaking, Mythra is designed for fast and quick combos to build up percent, while Pyra is a bit slower but much stronger when she hits. However, in practice, Mythra is all that and more. She's capable of racking up damage with her quick frame data, but is also plenty capable of knocking out opponents just as well as Pyra.
The key difference between Pyra and Mythra stems from guaranteed follow-ups and strong special attacks that can kill. Mythra's neutral special and side-special are both equally strong compared to Pyra, not necessarily in strength alone, but competitive viability. In the same vein of Melee's Sheik, Mythra is fully capable of setting up combos into guaranteed and 50/50 kill confirms. Pyra's knockout power is more closely dependent on reading the opponent, applying pressure, nailing edgeguards, and overall landing stray hits. Mythra can combo into kill setups, keep up the pressure with her fast speed, as well as utilize a very potent edgeguarding game with her up special.
Pyra is still a capable fighter, to be fair. It's just in comparison to Mythra, who by initial impressions has a much more versatile and useful moveset, where Pyra pales in comparison. Luckily, being the stance character she is, players don't have to decide between either. As the meta develops, undoubtedly players will find ways to harmonize the two characters quite well. For now, immediate impressions from numerous top players like Void, Mew2King, MkLeo, Marss, ESAM, and others have noted Mythra's impressive frame data and potential viability. Again, Pyra isn't as bad as Zelda compared to Sheik, but Mythra is comparatively more powerful character for Ultimate.
Super Smash Bros. Ultimate is available now on Nintendo Switch.