I'm glad you enjoyed it, and appreciate your feedback!
When you buy the upgrades, they have a picture of the sword or the hero next to them. Since you're the sword, you keep those upgrades forever. When your hero dies, you start over with a new one, so those are temporary. I was hoping that would be intuitive but it's always hard to get it right without iteration and testing.
I'm glad the progression felt good to you, I spent awhile tweaking hoping there was enough to motivate you to keep pushing forward, while grinding enough to avoid death.