Red Trainer

He lost to the Green Trainer. In the Pokemon Indigo League.

History
With the opening ceremonies behind them, Ash and his friends begin to prepare for the Indigo Plateau Conference. After registering, Ash learns that he must first pass through four preliminary battles on four unique battlefields (Grass, Water, Ice, and Rock) to qualify for battling inside Indigo Stadium. The lady at the registry has Ash go through a random selection, and he is assigned to battle on the Water field later that day against a Trainer named Mandi, who refers to himself as "Mandi the Astounding." Since he is assigned to the Water field, Ash plans to rely on Squirtle for the battle. Misty suggests that it might not be enough. The group runs into Gary, who is battling on the Ice field, who mockingly wishes Ash luck as he wants to be the one to defeat him, and he walks away laughing with his cheerleaders. At that moment, Ash receives a call from Professor Oak, who suggests he swap out one of his Pokémon. Ash decides to recall Krabby to boost his team. Misty is worried because Ash had never used Krabby before, but Ash is confident in the Pokémon. Meanwhile, Team Rocket is posing as a news crew to try to find rare Pokémon to steal. However, they were not having much luck. In one of their "Interviews", they come across Mandi, whose cocky attitude makes Jessie angry enough to nearly blow their cover. Before she can make true on her words though, James and Meowth remind her of their mission and that they should keep their cover intact. Ash's battle on the Water field soon begins. Misty and Brock get special seats behind Ash's podium on the battlefield to act as his coaches during the fights. After receiving a surprise good luck from Team Rocket, Ash starts with Krabby right away, and Mandi uses an Exeggutor. Krabby jumps into the pool, but Exeggutor uses its Psychic ability to create a whirlpool, revealing Krabby's hiding space. Mandi then orders an Egg Bomb, knocking Krabby into the whirlpool and preventing it from using any attacks. Ash tries to recall Krabby, but the whirlpool prevents him from doing so. Suddenly, Krabby escapes from the vortex, and Ash orders Krabby to use Vice Grip on Exeggutor's leaves, stopping its attacks. Hanging on tightly, Krabby uses Leer to immobilize Exeggutor and uses a Stomp attack to knock it out. Krabby stands triumphant over Exeggutor, and suddenly evolves into Kingler. Mandi, still confident, sends out his second Pokémon, a Seadra. Seadra uses Agility to dodge Kingler's Water Gun, but Kingler manages to slow it down with a Bubble attack and then knocks Seadra out of the fight with Crabhammer. Mandi, undaunted, sends out his last Pokémon — a Golbat. Brock worried that this could be bad for Ash; Kingler is weak against air attacks. Misty suggests Ash switch to Pikachu, but Ash stands by his Pokémon. Golbat manages to evade Kingler's attacks, dealing a great amount of damage in the process. But Ash remains firm and orders Kingler to use Hyper Beam. Kingler's shot hit Golbat dead-on, knocking it out and winning Ash his first Indigo League victory.