Great work! It was nice to ping pong between the knight and boss. The rogue-like feel spiced things up a lot. I ended up getting past the upgrade screen issue by resizing my window. (I zoomed out on my browser window a bit until I could click on the buttons)
There's definitely potential here if you wanted to add more power-ups and a long-term incentive for game progress. The sprites are also cute and appealing!
In terms of balance, I think the boss tended to be much harder to play as than the knight, but overall both are interesting complements to the other.