You Died features this symbol associated with the Sorcerer player character.
It’s shown in Dark Souls: Design Works in the section “Player Equipment Materials” for the “Sorcerer” under “Design Drafts” on page 90.
These Sorcerer design drafts look similar to the ones on the character classes poster.
The sorcerer in these designs looks quite different from what they look like in-game.
Maybe that’s the reason I haven’t been able to find this symbol anywhere in Dark Souls. It might have just been part of the design process.
It represents the letter G in the decryption key for You Died and Abyssal Archive.