Keith's eyes turns galra at certain parts in the fight. Seen in the images. He could have done this before but we couldn't see due to his paladin helmet and blade mask.
It could be that his galra instincts comes out during fights as a defensive mechanism or something like that giving him a bit of extra strength or it's starting to come out more as he's of age or something like that.
Maybe in a new episode we will see this more and he could turn more galra and go through a phase other then his emo one.
Because yes people are thinking his scar is a mark when it's not but something like that could happen, we saw his skin was purple when he got hurt so maybe he turns purple and actually morphs into a galra or only certain parts of him

What do you think?

Watch the fight scene again to see it.

Here's some clips but you have to really look at it

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