The Sword Maiden wears a blindfold, which usually indicates blindness. However, she does not act blind, nor does this wiki entry say she is blind.
If she's not blind, why does she have a blindfold?
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Yes, she is blind. In Volume 2 Chapter 7 of the light novel, it was indirectly stated that she is. To quote,
Sword Maiden didn’t answer. With her blind eyes, she stared at a person who seemed to her a shadow.
Also, in that same volume at Chapter 12, it was revealed how her eyes were blinded.
She had been shut up in a dark stone chamber, riddled with her own filth and that of her captors, crying piteously… They had burned both of her eyes with a torch. That was more than ten years ago now.