It's hard to know. If you look at the visual characteristics of Sasha's scar and the MC fight wound, they look the same (i.e. they aren't particularly convincing renditions). Sasha's mark still looks like a fresh wound rather than a scar, but it's never healed (unlike the MC wound), so it's presumably a scar. What could Ayua do in a fight that would produce such a scar, and why would Sasha need a bodyguard after a sparring accident with a friend?I always assumed the scar was due to sparring with Ayua. Ayua has a line where she takes a jab at Sasha about the last time they went at each other and Sasha touches her face. I think I remember it correctly. Of course, that could be a distractor.
Sasha has a hidden motivation to learn how to defend herself, make herself strong in that regard - I assume it's linked to how she got her scar (i.e. before she started taking lessons with Ayua).