Found all 6 mithril, beated yusha, didn't found the secret playable character yet.
Viewing post in Witch Hat & Ears of Cat comments
Oh, my bad! It's gonna be fixed in the next update.
I think I already replied to you on another comment, but the secret character is only available during one fight, and only if you did something right beforehand. That's all I can say ;)
Also congrats! You may just be the first person to beat the secret boss! xD How difficult was it?
The reward is L O R E.
For for real, I wanted to give you another mithril piece of smth, but if you beat the hardest fight in the game, what can I even give you? xD
If that's worth anything tho, after you recover the mithril from the Fire Temple, you can go ask the fire spirit about the dragon statue there, and then talk to chief Eino. You may gain some explanations on what is going on there.