Hey! Glad you liked it.
As for that training scene with Katharine — the protagonist only loses health if he runs out of stamina or fails to block the attack.
That said, there are plenty of issues with that scene, and the main one is the Ren'Py engine itself, which eats up CPU like I'm running a space simulation.
You can check it yourself :D
Even so, I still found it confusing even with the explanation. I got past the defense part, I don't know how. In the attack part, I found it quite confusing, because even if you swing the sword quickly to do double damage, there are times when Katherine doesn't take the damage, even if you do what the explanation says. But I'm looking forward to the update. I've already added it to my list of games I've played.