Dipplin and Hydrapple were both introduced as new evolutions of the Applin line in the Pokémon Scarlet and Violet DLC, and Kieran might’ve been the one to discover that final evolution according to one fan theory.
Kieran’s character arc and Applin’s new evolutions were two big aspects of The Teal Mask and The Indigo Disk DLCs. Through the two-part story, we watched as Kieran went from being the shy kid to becoming the arrogant BB League Champion, motivated to be better than the player. During this period of growth for Kieran, his Dipplin also evolved into Hydrapple at some point after The Teal Mask and before The Indigo Disk. According to an interesting headcanon in a Jan. 15 Reddit thread, this could’ve been when Hydrapple was first discovered.
The headcanon talks about the specific circumstances involved in the evolution process for Dipplin and Hydrapple, and they do align with Kieran’s background and story. Currently, Applin’s evolution into Dipplin is tied to Kieran’s homeland, Kitakami. The apple stand in Kitakami’s Mossfell Confluence is the only place to obtain the Syrupy Apple, the item required to evolve Applin into Dipplin.
At this point, Hydrapple wasn’t in the Gen IX games yet, though an evolution was unofficially confirmed due to Dipplin’s interaction with the Eviolite. Perhaps Hydrapple wasn’t even discovered yet by people in the Pokémon world. Otherwise, Kieran would’ve been the type to evolve his Dipplin as soon as possible in an attempt to outshine the player.
We see Kieran finally has Hydrapple by the time The Indigo Disk starts. To evolve Dipplin into Hydrapple, you need to use TM226 on it to teach it Dragon Cheer, and that TM is currently only distributed by Drayton at Unova’s Blueberry Academy—Kieran’s school. So Kieran essentially brought his Dipplin (which requires a Kitakami-exclusive item) with him to Blueberry Academy (the only place to obtain TM226 Dragon Cheer). In other words, Kieran checked off the two location-specific requirements to get his Applin evolved up to Hydrapple.
By pure coincidence, he might’ve wanted to teach his Dipplin a new move to make it better in battle and accidentally discovered the Hydrapple evolution by teaching it Dragon Cheer. This would make sense lore-wise, considering Kieran was at a point where he was obsessed with becoming stronger, and his Dipplin couldn’t learn any new moves after level 44 unless he used TMs, as pointed out by the OP.
Although this is just a headcanon for now, it would explain why Dipplin and Hydrapple weren’t introduced at the same time; Dipplin debuted in The Teal Mask, while we had to wait until The Indigo Disk to meet Hydrapple. An interesting theory, and one we may see explored in the future.
