The DLC for Xenoblade 2 allows Mythra to fight on her own. Pyra didn't exist at that point in the story, since the DLC is a prequel.Oh... and to some of the above comments...
Min Min is absolutely the main character of ARMS. Just gonna throw that back out again.
Or rather, she is the mascot for it- aka, the most popular, known character.
The main characters for Tekken follow the entire Mishima line.
I didn't ever understand Pyra/Mythra, though. In the games- from my viewing- it looks like they never fight, they just are connected via energy to Rex. Is this incorrect? If so, I don't understand why Rex isn't the fighter, with Pyra or Mythra in the background, like Pokemon Trainer.
Sakurai told us why he didn't do Pyra/Rex, because having 8 players on the screen at once with Pyra and Rex would be too much. Pyra is much more involved than Pokemon Trainer ever could be in fighting.