Gantetsu

Anime: Ash was sent to Johto to deliver the GS Ball to Kurt for study and found Azalea Town suffering through a drought.

Ash accidentally stepped on the tail of a local Slowpoke, and he was chased by an angry mob, but Kurt disguised him and his friends in Slowpoke costumes and saved them.

Kurt lived in Azalea Town, a town that considered Slowpoke sacred for their mythical abilities to cause rainfall. Ash and his friends met him while investigating Slowpoke Well because he heard strange noises coming from within. It turned out that Team Rocket was trying to steal the Slowpoke. Ash helped recover the Slowpoke and defeated Team Rocket.

Later, Kurt received the GS Ball. Ash wanted to see if Kurt would make him special Poké Balls. Kurt gave Ash, Brock, and Misty each a Fast Ball as thanks for helping them.

Ash and his friends gathered Apricorns and Kurt made Ash and Misty Lure Balls, and made Brock a Heavy Ball.

As of the latest anime episode, Kurt is still studying the GS Ball.

Manga: Silver got a Heavy Ball from Kurt, intending to catch an Ursaring with it. Kurt cautioned Silver that only experienced trainers could wield his special Poké Balls.

Later, Gold met Maisy and Kurt. Kurt explained that once, his ancestors were the only ones capable of creating Poké Balls until companies started to mass produce them. Kurt then carried a slight grudge against Gold for making fun of his Apricorn Balls, saying that Gold wasn't experienced enough to use his special Balls, something that annoyed Gold. He then challenged Gold to catch a Pokémon using one of his balls and if he couldn't by dusk, he would have to become his apprentice, which he accepted.

Kurt made a Friend Ball for Gold, and later Gold and Silver were able to use their custom Poké Balls to capture a Teddiursa and Ursaring. Silver kept Ursaring for himself while Gold gave Teddiursa to Maisy. Kurt then appeared at Slowpoke Well, to see a mass of Team Rocket Grunts tied up and defeated, everyone wondering who did it.

Later, the Masked Man forced Kurt to turn over his plans for the GS Ball.

In their youth, Professor Oak, Agatha, Kurt, Pryce, and the old couple who run the Goldenrod City Pokémon Daycare were all close friends. To try to cheer Pryce up, Kurt helped write the lyrics of "Riding On The Stars," which was also sung by DJ Mary after that.

(Source: Bulbapedia)