Mira is a main villain in DBO and the Xenoverse games, as well as a major character in Heroes. Basically, he's an artificial demon created by Towa (Dabura's sister) from the DNA of multiple fighters in history and helps her cause shenanigans across history via time travel. In other words, he's overgrown Smurf Cell.
To fight him, you need to defeat all the Villainous enemies (the ones with red aura that are stationary in the sky), which you can't do until the post-game. Once you've beaten all the ones available, Villainous Gotenks & Vegetto will appear in East Ravine Area. After you beat them, you can fight Mira on the Kaioshinkai.
His appearance in the game essentially bridges the gap between his actions in Xenoverse and Xenoverse 2, as well as clears up some of the differences to the manga canon by implying Kakarot may simply be an alternate timeline to some extent.