This game could've been something really fun, as I generally enjoy action-platformers. In fact, Super Ghouls 'n Ghosts, known for its difficulty, is one of my all-time faves, as are games like Shadow Dancer. I even enjoy Hagane, seen as one of the more average in the genre (incredible sprite-work tho). That being said, this game made several questionable decisions in terms of mechanics, physics and design that bogged down the experience for me.
Firstly, as others have mentioned, the auto-adjust difficulty feature just doesn't work out so well. Most games that implement it make it so difficulty gradually increases as you keep doing good. However, those games don't tend to be platformers, and for good reason. The purpose of challenge in platformers is to have the player learn how to navigate across perilous obstacles. If you implement a feature so where that obstacle is automatically removed after the player dies, you are removing a core component of the platforming experience. At the very least, implement a feature allowing players to turn it on or off. It reminds me VERY much of Nintendo's Super Guide feature w/ their new Mario games. I dare say you have a better interpretation of that concept, but the fact it is forced universally on all players no matter the difficulty level completely negates its positive attributes. As someone who cuts their teeth on rising to the challenge of a fair-difficult platformer, salivating to be able to clear that nagging leap of death successfully, it feels like I've just been mugged twice in one night after dying, and coming back to see I can't try a different approach b/c the obstacle is gone.
I also have several other issues. The lack of a look-down/look-up ability would be forgivable in an easier platformer, but not this one. There are too many instances of blind jumps and landings to be had, and the ability to pan the camera a bit after holding up or down would have been very nice. The only way you can get over this is to explore other parts of a stage and hope you get a better view of a section you need to traverse next while there. That may be encouraging exploration, but not in a fair way and not in a way left to the player's discretion.
The jumping physics also need work. Jumps here feel too floaty; it's almost as bad as it was in Virtua Fighter 1, which is infamous for its spaceman jumps. To be more precise, it never once felt like I could get a solid small jump, as even slight taps would send me just pixels too high and nearing collision w/ spikes. The game could have also benefited with a separate Jump button too, which should be mandatory in a game w/ as much focus on precise jumping as this one.
Collision detection is the final nail in the coffin. I did not notice this much in Levels 1-4, but by Level 5 it was plainly obvious that the hit detection for some enemies expanded well past their sprites. Coming up on a robot and being killed by its invisible gas leaks is about as douche a death as it gets. Points for realism, but certainly not for fun.
Regarding pet peeves, I'd of preferred a checkpoint system and perhaps a hit-point system; not grossly massive in scope (maybe just one checkpoint per stage and two hitpoints), but it could've gone a long way. Perhaps making the player a tad faster would be nice as well.
I generally liked everything else, such as the visuals, music etc. So kudos on that.
What you have here is a 'rough' blueprint. I feel you can get a real gem polished w/ a fine-tuned update, but as of right now it is merely mediocre. It does pull on my bias nature in favor of these sorts of games, but against even the average in the genre this one has technical and design flaws that chew it out pretty bad. But, I look forward to seeing the improvements you make in future iterations, hopefully taking my and legitimate criticisms from others to heart.