Very nice demo! I have 3/4 secrets, but that last one I'm at a loss for. There's a patch of green platform-y grass under the ground in a map area nearish the start, but as far as I can tell that's just a bug resulting from mislayered textures for the nearby dead tree that's part of the same tile. There's also the log in the ghost area which, if it is moveable, I have no idea how to move.
I quite like the different movement options especially the air-hook uppercut thing, although that's presently useless lacking moving platforms not to mention a downwards aerial attack. The base run speed is a bit slow, and the pegasus dash hardly helps since you lose dash state when attacking, but you can quickstep backwards across maps pretty quickly if you really want to and I can put up with the slow pace mostly. It's most annoying when backtracking through previously explored areas looking for secrets, because although the movement is fun when dealing with obstacles enemies are quickly not a threat (except for the lack of healing options making even trivial damage annoyingly persistent if allowed to accumulate), yet still take a long time to deal with just because of the self-hitstun and inability to attack and move/dash and in some cases the elevated spawn rates. It really is a minor issue, though; the game is quite fun overall.
If there's a way to beat the boss without just tanking a bunch of hits, I don't know it, but tanking a bunch of hits worked fine. He's a bit tall to jump over without taking damage, and his attack where he briefly goes underground both doesn't have him underground for very long and doesn't have any way to trigger it that I can tell.
I had fun with the secrets so far, but would appreciate more. Like, it would be cool to have a secret area accessible from at least one of: the Pegasus feather room, the dash boots room, the secret hp up room near the mattock. Maybe a combat trial or an obstacle course or something.
Anyhow, thanks for making this and good luck on the next update.