A permanent stamina increase on Pebble would be extremely overpowered. Everyone would just use him repeatedly to boost stamina, and once his max stamina gets high enough, it would make all the other turtles obsolete.
Personally I feel that if we add a permanent stamina increase to Pebble but then cap it at a certain point, it might be better to just slightly buff Pebble's base stamina instead. Otherwise it could end up feeling like an unnecessary grind just to reach what's basically the same result.
When I first designed Pebble, the idea actually was to have permanent stamina increases. But after thinking it through a lot more, it just doesn't really fit Serenitrove's gameplay loop, where every run is meant to start fresh.
it's tiny