With last December's release Pokémon The Movie: Secrets of the Jungle including a direct reference to Ash Ketchum's father, speculation is once again rife among fans about who the mystery person may be. As theories continue to spread, one often overlooked part of Ash's journey is that he actually had a father figure the whole time: Brock.
From their very first meeting at the Pewter City Gym, Brock has taught Ash a number of important lessons about Pokémon and about life. He was a stable presence and guide for Ash throughout multiple seasons, particularly the Indigo League era when Ash had just left home and knew only the bare minimum about Pokémon training. Though the two aren't related by blood, Brock's nurturing presence and patient guidance did plenty to help Ash grow, his influence even complementing that of Ash's mother Delia.
An important thing Ash and Brock have in common is fathers who left their family to become Pokémon trainers. But while Ash is an only child, Brock comes from a large family. When Brock is first introduced in the series, he's the Pewter City Gym Leader but spends most of his time taking care of his nine younger siblings at home. He fulfills a crucial parental role by doing the cooking, cleaning, sewing and laundry for ten -- all while holding down a job at the same time. Brock's father Flint eventually returns home to take care of the family so Brock can travel with Ash, but Brock's time caring for his siblings taught him a number of invaluable lessons about life -- plenty of which were passed on to Ash and Misty.
When a confident but naïve Ash challenges Gym Leader Brock a second time, his powered-up Pikachu is still outmatched by Brock's Onyx. Though Pikachu is in serious trouble, Ash is too stubborn to concede defeat, leaving Brock to call Onyx off and end the match himself. Ash catches a lucky break, in the end, thanks to the Gym's water sprinklers weakening the Rock-type, but Brock's actions stopped Pikachu from being permanently injured. This taught Ash the importance of humility and putting a Pokémon's well-being above all else -- even winning Gym battles.
Brock brings a level of life experience Ash didn't have yet. As their journey goes on, Brock is often shown looking after Ash and Misty like a father with two kids. He does most of the cooking and cleaning, stays calm in dangerous travel spots, battles Team Rocket and often scolds Ash and Misty whenever they start fighting. His Pokémon breeding studies and techniques also teach Ash how to care for his Pokémon's health, diet and general well-being, forming even stronger bonds with them.
Like Delia, Brock criticizes Ash for certain mistakes but always mixes it with positive advice and encouragement while celebrating his accomplishments. And like any good parent or friend, Brock helps keep Ash grounded whenever he gets a little cocky. It's fitting that when the trio gets lost or are trying to find their next path, it's usually Brock handling the map and showing the others where to go (sometimes after getting them lost to begin with). He's their guide, consistently making up for what Ash and Misty, both still very young kids and trainers, are missing while their parents aren't around.
His immaturity gags around certain girls aside, Brock very much fills the role of a caregiver for Ash on his Pokémon journey. Ash may not have seen his paternal father for a very long time, but he's had a father figure and friend teaching him and helping him grow all the while.