I think Saix got it when he lost his Heart. Those tattos under Axel's eyes, I think he got those from when he lost hie Heart too, but just painted over the scar. Yeah, I think Saix & Axel joined the Org at the same time; Saix just got luck as was number VII.
my theory is that saix (along with axel)and leaxeus were heros (reson frome part of thiere nick names saix "diviner" and leaxeus "hero")and frome one of thire epic battels saix got his scar