I loved this game. It reminded me a lot of Phoenotopia in a weird way. Though the castlevania and megaman references are pretty blatant. And that last fight. Let's just say the evasion subweapon is OP. Like, really OP.
1. In the end-game boss rush, I kept dying if my health went below 5,000. It would become NaN. This is a programming mistake that can easily be avoided by being more diligent with your handling of types. Try using strict type equality (=== and !==) instead of value-only equality (== and !=), for instance. Oh, and use isNaN() to make sure a value that you expect to be a number is always a number. Remember, NaN isn't equal to anything, not even itself.
2. When I defeated Not-Sans, I hit her right before her dialogue popped up. This resulted in the entity being erased, I guess, because after she spoke her dialogue, I couldn't hit her at all (because I already did) and was basically stuck in limbo. I don't really feel like doing that fight again to see what happens when you don't accidentally hit her at the wrong frame and get stuck in limbo, but I thought I'd let you know.