I have things to comment about, as well, but I think cicadahuman outlined most everything.
- Music is great.
- Graphics are good.
- Jumping makes sense considering you're a vampire and have increased strength, but jumping over half the screen height feels odd, as well as how slow the jump is.
- Forward momentum is wonky; it almost feels like you're moving in slow motion. This, also, adversely affects the jumps and makes platforming more difficult. I think you should turn off acceleration or make it max.
- When you swing your whip, it takes a moment before the projectile is released to do damage. I've ran into several occasions where I'll whip an enemy close to me and my whip passes through the enemy, it hits me, then it dies. Also, there's too much of a lag time between swinging again.
- Bats appearing out of nowhere and moving in a straight line is odd.
- I think that the whip should be longer (if not from the start, upgradable).
All-in-all, though, great job. I'm working through a game in GB Studio, as well, and it is quite the chore to get everything adjusted JUUUUUST right, so great work on what you've got thus far. Looking forward to seeing more of the game.