That question's got layers, doesn't it? I've read a bunch where the pokemon's intelligence and personality get developed so far beyond the games—sometimes they'll use a character like a Gardevoir or Lucario, something humanoid, but the interesting ones are when the author goes with a less obvious choice. A trainer and his Growlithe learning to communicate without words after a traumatic event, that sort of thing. The growth isn't just about getting stronger for battles; it becomes about mutual understanding, overcoming instinct, and redefining what a partnership means.
It can get pretty philosophical, actually. When a character has to care for a creature with a completely different worldview, it forces him to examine his own assumptions about strength and companionship. I remember one story where a trainer's Ninetales, through a psychic link, forced him to confront his own guilt over past losses. The pokemon wasn't just a tool for his growth—it was the catalyst and an active participant. The dynamic shifts from owner-and-pet to something way more complex, and that's where you see real change in both characters.
Honestly, the execution varies wildly. Some fics handle it with surprising subtlety, while others... don't. But at its best, this niche uses the fantastical premise to ask grounded questions about responsibility and connection.