When Tatsuya learns of Mizuki's sensitive eyes, he thinks to himself that he has to take care not to let her discover something about him.
Unmarked spoilers below.
What was it that she could discover with her ability?
- The seal Miyuki placed on his abilities (since mantaining the seal uses up her magic processor, it is a kind of ongoing effect).
- His special abilities. Like she could sense his use of elemental sight.
- The fact he was not a born magician (his mother used a magic surgery to implant a processor in his brain).
- That she could catch him using flash casting
- Something else?
Also, if the source material can be referenced (like the novels or something), all the better.