The execution on this is fantastic, you really covered every element of concept design well. Most people have a clear weakness in lore writing, or number balancing, or whatever. But you actually tackled everything well. The voice acting and theme music worked better than I expected(but as a note for the future, let your voice out a bit more. The "kinda whispering but kinda shouting" doesnt sound natural)
I appreciate that this is a simple concept. As much as I love making crazy and completely game changing concepts, every once in a while I'll just do a concept without any gimmicks or new mechanics, and just try to make a kit that works well. The shield breaking is a new mechanic, but its pretty straightforward, and is also a nice counter to a mechanic that currently has no specific counters.
My only real critique is on his identity. His backstory is well writen and expresses his character really well, but I dont know if it does enough to distinguish itself from the countless other "gritty, master swordsmen with dark pasts". And I'm not just talking about Yasuo and Riven, I mean in general media as well. I'll admit that I'm biased, as I personally prefer experimental and creative archetypes over more universal and reliable archetypes, but its still something to consider
On a more minor note, Q damage is really high at level 1. If both hits land, its ~180+4% max health damage, plus the shield as a bonus.