itch.io

the hint for the 6th character is in the frame where Dust is thinking and there's a bunch of images flashing by. You can see a 6 above most of a 2. 

At first I thought it was a question mark (? and 2 can look very similar when the bottom is cut off), but I realized what it was when I found most of the other letters.

This was a great game, and the secret was pretty cool too. I love puzzles, and this one was just the right difficulty to crack without having to set aside too much time while also being incredibly satisfying.