I like it! The jump controls are really fluent and reliable, the 'rewind' after death is a nice touch.
It's a pity you didn't create more than six levels, though. The last two were really more about speed (and luck) than about your platforming skills, so my favourite one is probably the fourth one which requires you both to be precise and fast, but a tiny mishap doesn't automatically mean you'll be killed.
I really think the level should only begin when you move, and not sooner. Like this, you have to start moving immediately after the level loads to have the most time possible, which is absolutely crucial (and annoying) in the last two levels. If the shadow only started moving when you do, this inconvenience would go away.
E.g. while writing this review, I'm constantly being killed again and again just because there's no pause and the level commences at once. Also, you have a level select screen, but no way to return to it once you open a level (unless you beat it).
A few nitpicks:
– Your shadow remains even after you blast into a puff of smoke (it should disappear, too)
– When you're about to die (killed with 1 life left), I wouldn't even rewind back, just puff instantly. Otherwise, it makes you watch the rewind and even move for about a half a second until the game finally kills you off.
– Is there really a need for an FPS counter?
– You also picked a monochromatic colour palette for the game (which is a good idea for a light/shadow themed game), but the puff of smoke and the life bar are pink, the level restart text is purple, not to mention the menu. Wouldn't it be better to stay within shades of black and white?
But what this game truly needs are more levels. You could introduce new features as the levels progress, too (for example a key you need to pick along the way to unlock the door, spike traps, moving platforms, etc).