Clemont.

Clemont is the Lumiose City Gym Leader. His father Meyer runs a Lumiose electric shop; Clemont and his much younger sister Bonnie live above it. Clemont specializes in electric-type Pokémon and inventing; his "Clemontic Gear" gadgets are introduced at least once per arc, almost always with a disclaimed safety brief, almost always exploding. The visual gag is consistent and the joke ages well.

He meets Ash early in XY003 when Ash visits Lumiose for the Gym Badge; Clemont is locked out of his own gym at the time (a robot caretaker has taken over) and joins Ash's travels to figure out how to reclaim it. Once he does (in XY067), he formally challenges Ash. The Lumiose Gym battle is one of the era's standout matches; Clemont's Heliolisk and Luxray push Pikachu and Ash-Greninja to their limits. Stays with Ash through the rest of XY/XYZ, then returns to Lumiose to run the gym alongside Bonnie when XY141 closes.