Thanks for playing! I guess more clarity is needed.
All of this is potentially temporary. Things may change as development continues but right now collecting allies restores the most amount of stamina after the level is finished.
Walking through the dungeon slowly reduces the available stamina and the stamina upgrade prevents it from draining so fast.
Hitting enemies greatly reduces a big chunk of your stamina and collecting the healing items restores a small segment of your stamina.
The general idea is that with every level completed, the levels get bigger so you'll need the upgrades to reach the end without running out of stamina. If you do run out of stamina then the only way to get game over is by hitting an enemy, so it's not 100% necessary but gives extra insurance against failing.
I'd like to add in some items, more enemies, and some more variety in general. Maybe also change up what collecting allies does. It's all open ended at the moment.