Shiina's birthday is two days before that of Yaguruma Saburou, and the two are childhood friends. 
She has overly acute hearing due to her magical perception. Magical researchers who have diagnosed her have presumed that, "she always uses magic that strengthens her hearing unconsciously." Adjusting her hearing with magic impairs her sense of magic and hinders her use of it. To almost shut off external sounds and adjust the volume automatically, she always wears earmuffs with a built-in microphone and speaker. Even if she wears the earmuffs, it doesn't hindrance her use of magic, but it becomes insensitive to external magical waves.
Due to her training since childhood at the Third Institute, where military magicians have come and gone, her combat strength is high.