When you step outside the stadium after claiming your 4th Gym badge, Sonia will come to find you. Suddenly, you're interrupted by a loud noise. Head west, then follow the stairs all the way up to the Historic Site. At the top, you'll find the perpetrator is none other than Bede. Approach him for a battle.
Pokémon Trainer Bede
With the exception of Ponyta, Bede has evolved all of his Psychic-type pals. If you have a Dark-type, this battle is a piece of cake. Duosion can use Reflect and Light Screen to protect its team from physical or special attacks. The other three are pretty standard fare, although Ponyta and Hattrem do carry Fairy-type moves.
After his defeat, Bede will be escorted off the scene. Then you'll learn a shocking (or maybe not) secret about the Galar region's history. As thanks, you'll receive 2 Revives from Sonia. Now go back down the stairs, then head east from the stadium, towards Glimwood Tangle.