I'm thinking more when WB got his chest scar...
Clearly not this chapter.
So who did such.
The only answer I can think of is "Lurking Legend". And he's the one who Shanks came to talk to the Gorosei about to warn them about his danger.
And he must be related to Shanks.
At first, I thought it's Roger, but clearly Roger wasn't the cause for the scar.
Also, saying that "seeing your face reminds me of the scar that guy gave me" makes it more personal for Shanks. By personal, I mean it makes it more "blood-related". Someone like a father or an older brother...etc.
ORRR, it's Buggy's alter ego LOL
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I found 1 interesting theory that it was Kid BlackBeard who gave WB those scars
But how? why would WB keep him on the ship?
Or is it that BB has some kind of monster who took over his body? maybe he has more personalities and one of them was that?
But it wouldn't make sense for WB to say that "he is the only one he cannot forgive" during his battle with him, if Blackbeard simply had someone take over his body, and WB knew about it, and kept him on the ship, and then later he said he cannot forgive him. Feels very contradictory for WB personality