Character scars

Matt has a few scars, one - point-five canon, and one headcanon.

Matt's first, and most obvious is the one on his cheek that happened sometime between Kerberos and when Pidge found him.

I'm pretty sure it happened in some battle. He doesn't really talk about it.

The second is... his knee. There's a huge scar, just under his right knee cap because of Shiro in the Arena.

He wasn't able to take care of it well after it happened, so it looks worse than it should.

(That's the point five canon scar. We don't ever see it, but you can't tell me that there isn't something there after what happened.)

Third, I headcanon that he has whip scars on his back from his time in the workcamps/prison. Matt's a mouthy one, and he wouldn't keep his mouth shut around the Galra. So he almost certainly got beat. A lot.

(He used to be a space adventurer.)

Given Shiro's strength and the fact that it was a sword, it wouldn't make sense that he wouldn't have a scar on his knee.

Exactly.

We just don't see it in canon, because Matt's always wearing something that covers his knees.

Carlos has a scar on his calf from where one of his mother's bear traps (that were protecting her furs) cut him when he was younger. (Yes, this is book canon.)