The piano. Wow. Not sure there's much I can say about it that will do it justice so I'll just say the playing is outstanding. Though I'm almost positive I've heard the exact melody at 3:04 in some anime. Even if I haven't, it - along with some less serious offenders - is fairly generic in composition.
The singing is somewhat hit or miss. I like that you're not belting - you have a good mix of being soft and resonant, the harmonizations sound excellent, just the right amount of vibrato, but you sing a bit flat at times and hearing you draw in breath is distracting at certain points. There are also a couple points (not consistently) where I can hear some vocal tension that makes you sound a tiny bit strained.
I don't want to say much on lyrics - years of rapping have made me obsessively picky about rhyme schemes and complexity, and without having a firm grasp on what this song is really about I think it's fair for me to try and point you in one direction or another.
Taken as a whole, this is a really pretty piece. Nothing about it strikes me as really groundbreaking, but there's an emotional depth to it that's very alluring. A few quirks with the vocals broke my immersion once or twice but this was genuinely a great listen.