Flint is Brock's father and the former leader of the Pewter Gym.
Season 1: Indigo League
Ash came across Flint when he and Misty had just arrived to Pewter City. He welcomed them to Pewter City by informing Ash that he was sitting on his merchandise. Ash was later charged $2.00 for doing so. When Misty informed Ash of the Pokémon League, Flint underestimated his abilities. However, he later welcomes him to his house, and comforts him for his lost Gym Battle with Brock. He took Ash to his house and showed Brock taking care of his eight younger brothers and sisters. He also explained that Brock's mother was sick and his father left home to fulfill his dream, only to fail and hide away in shame. He then helps Ash, and Pikachu by bringing them to an old power plant, so that Pikachu can charge up for the rematch battle. When Ash unofficially defeats Brock, he reveals himself to be Brock's father. He allows Brock to leave with Ash to fulfill his dreams.
Season EX: Pokémon: Heroes
During Brock's time away from home, Flint allowed his wife, Lola, to redecorate the interior and exterior of the gym. Despite badgering his son to come back home to visit, he tries to chase Brock away, knowing his reaction. He fails to get rid of Brock and allows him to see the inside and outside of the redecorated gym. Flint also leaves out that Lola has filled the gym with water for her Pokémon to idle in. He explains to Brock that he tried to desuade his wife from her actions, but she decided not to listen to him. Since Flint is the new gym leader, Brock forces him and his mother to fight for the gym. However, because his mother has all water-type Pokémon and his father has all rock-type, Lola wins.