From the Friction page in Wikipedia
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.
Dry friction is a force that opposes the relative lateral motion of two solid surfaces in contact
Hmmm, dry substances are supposed to have significantly less friction, HMMMM
Also air resistance is supposed to be less than friction, HMMMM
Also also the floor is literally brick, HMMMMM
Also also also if the butter is melting and reducing friction then it should be dying, HMMMM
Jokes aside, the theming is very good! I like how the idea of a samurai in a Japanese-themed environment, and the music is quite good as well, great pixel style of music, I like it.
A cool idea I thought of is making the game easier but in turn making the character melt over time, reducing friction, making it even more stressful. Just a dumb idea though, as not making a time limit in rage games tend to work better.
The physics is quite weird. The game feels less like you're controlling a slippery character and more like the floor is ice/slippery, most notably seen when you speed up after touching the ground for some reason. The two are definitely different, in game feel-wise if the character is slippery they should be shown that way when jumping and stuff. I can't really explain exactly what to change, just note that if your character is supposed to be slippery/fluid they should maybe feel a little different in the air or something(the current game is like fighting against the environment, instead of fighting with difficult controls/character, both work but in your case you're trying to make the character the problem)
Making the character feel fair is very important. For example, you can increase acceleration but reduce max speed a tiny bit so it you can change directions faster, something in the lines of that. You can keep the nightmarish deceleration. This won't exactly make the game easier, but it would make it feel more fair.
Try making the player hitbox just ever so slightly wider, even bigger than the sprite. This will make the game have a better feel, it even has a term, known as "coyote time". Currently the character feels rather clumsy.
Anyways, quite a polished game! I love the idea. I would love to finish the game, however it would take maybe 30 minutes, and I don't quite have the time now. I might revisit it later.