The more I look at the letters in the image, the more I'm convinced it once said One Piece.
You can see part of the O and N and the middle line of the E on the left side. Then a space and what looks very much like the anchor that is often displayed in the middle of the One Piece logo. We've been thinking it was a Z or L, but I don't think so any more. I'll attach a close up below.
What looks like the A on the right could be the P, and the last letter definitely looks like the sideways anchor making a curvy E (not a G) that One Piece uses. The other letters in "PIECE" - I, E, C are tough to fit in there, but the I is usually replaced by the image of a character in their logo, and might not have been embossed onto the sole at all. That leaves E and C, which would fit in the remaining space, and could be an imperfect impression in the print.
I've yet to find a matching sole, but I've learned that a lot of anime fans like to do their own art, especially on their shoes. There are inexpensive blank soles and engraving techniques out there that could have been used.