Clemont is the leader of the Lumiose City Gym. He uses Electric-type pokémon and awards the Voltage Badge.
Gym Leader Clemont
Clemont has a level 35 Emolga (vulnerable to Rock and Ice moves), a level 35 Magneton (it has a Steel move, so don't send Rock-type pokémon against it), and a level 37 Heliolisk. The Heliolisk has a Grass-type move, which is strong against Rock and Ground pokémon, so be careful! As mentioned earlier on this page, I recommend catching a Gible in Route 13: Lumiose Badlands and using it against Clemont's Heliolisk, because Grass-type moves aren't super-effective against Gible.
When you win, Clemont gives you the Voltage Badge, which causes traded pokémon up to level 70 to obey you. Clemont also gives you TM24 Thunderbolt.
When you go outside, you receive a Holo Clip. Professor Sycamore asks you to meet him at the Lysandre Café. To get there, first go down Autumnal Avenue, then go into the red café to the left.
In the café, walk toward Professor Sycamore and Lysandre. They were having a discussion about Mega Evolution, which Lydandre would like to be able to use. You learn that Lysandre is possibly a descendant of the king's younger brother, although that story is 3000 years old so it might not be accurate.
The Professor says that Lysandre's Lab makes the Holo Caster, and Lysandre uses some of the profits to help pokémon and trainers. Lysandre wants to be the type of person who gives, not takes. He speaks of a terrible weapon that the former king of Kalos made. Lysandre thinks the weapon was bad, but on the other hand he thinks it was good that it got rid of the "filth".
Lysandre gives you a King's Rock. If you give it to Poliwhirl, it will become Politoed if traded. If you give it to Slowpoke, it will become Slowking when traded. Also, if a pokémon holds it in battle, that pokémon has a 10% chance of causing flinching when it uses an attack move that doesn't already have a chance of causing flinching.
When you leave the café, you get another Holo Clip. It's from Trevor, who asks you to meet up with him and your friends in Route 14. When you're ready, you should leave the city and go to Route 14: Laverre Nature Trail.