Muno

Flint first appeared in Showdown in Pewter City. Ash and Misty met Flint, who was selling rocks as souvenirs. Ash bragged about his goal to become a Pokémon Master, but Flint told Ash that thinking he could defeat Brock, the Pewter City gym leader was foolish.

After the match, which Ash lost, Flint brought him home and talked with Ash about Brock. Flint took him to Brock's house and revealed that Brock had nine younger siblings that he constantly had to take care of, because his father left to become a Pokémon Trainer. He never returned, as did his mother.

Flint decided to help Ash in his gym battle and hooked Pikachu to a hydro-electric dam to increase its strength. After the gym battle, Flint showed up and revealed himself to be Brock's father. He had failed to become a successful Pokémon Trainer and couldn't face Brock and his siblings. As Brock wanted to travel with Ash, he left the Gym Leader position and the duty of being head of the family to Flint.

Flint met Brock again in A Family That Battles Together, Stays Together! when he returned to Pewter City after traveling through Kanto, Johto, and the Orange Archipelago. Brock scolded Flint for not taking care of the Gym. An embarrassed Flint explained that that he had just got the Gym set up the way he liked it when Brock's mother returned. Flint took Brock back to the Gym, which had been redecorated. Flint closed it because he couldn't stand it anymore.

Lola and Flint had a rematch for the title of Gym Leader, which he lost. Fortunately, Brock was able to beat his mother. Flint and Lola then agreed to change the Gym to specialize in a combination of Water-types and Rock-types.

It was revealed that Flint gave Brock his first Pokémon, an Onix, for a birthday present.

(Source: Bulbapedia)