Cilan.

Cilan is one of three Striaton Gym Leaders: a triplet with brothers Chili (fire-type) and Cress (water-type). The Striaton Gym chooses its challenger by matchup; whoever has type advantage battles. Cilan additionally holds an A-Class Pokémon Connoisseur license, an in-universe credential that judges Pokémon-trainer compatibility. He joins Ash and Iris after the gym battle because Connoisseur evaluation requires field study and the Ash-Pikachu pairing is, in Cilan's assessment, fascinating.

Defining quirk: connoisseur monologues. Cilan tastes every situation - a battle, a meal, a relationship, a Pokémon move - and delivers a wine-sommelier-style assessment. The bit could have been one-note; the writers vary it across food, fashion, philosophy, and battle tactics, and it ages well. By Decolora he's also picked up Fishing Connoisseur and Detective Connoisseur certifications, ostensibly to make every locale shootable as an episode-of-the-week. Catches Pansage (his signature), Crustle, Stunfisk. Departs in BW144 to study additional Connoisseur disciplines.