Brock.

Brock is the eldest of ten siblings. His father Flint walked out and his mother followed; Brock raised his nine younger siblings while running the Pewter City Gym. When Ash defeats him for the Boulder Badge in EP005, Flint reveals himself, takes the Gym back, and Brock leaves with Ash to pursue his real ambition: Pokémon Breeding. He travels with Ash longer than any other companion (Kanto, Johto, Hoenn, Sinnoh) and through the Diamond & Pearl arc shifts his goal from Breeder to Pokémon Doctor.

His defining comedic beat is falling instantly in love with every Nurse Joy and Officer Jenny he sees. Misty's Psyduck pulls him away in the original series; later it's his own Croagunk (introduced DP005) who Poison Jabs him out of the scene. The bit never gets tired because the writers know exactly when to pull it back. His competence as a Breeder, Trainer, and later Doctor is also established repeatedly; the comedy works because it sits on top of genuine expertise.

Leaves the main cast in DP191 to study to become a Pokémon Doctor (a goal he formalizes after the Sinnoh League). Returns for the Aim to Be a Pokémon Master epilogue alongside Misty. By that point he is a fully credentialed Doctor; the show implies he runs a clinic somewhere in Kanto.