TOH
If he wasn't someone she could rely on, someone she felt safe with. He doubts their relationship would have progressed this far. From the very moment they met each other. It was one of the things he noticed about her. If she is around people she feels safe with, she can act more like herself. When she is around those she is uncomfortable with she regresses into a colder version of herself. A version that actually resembles how Terashima is with most people. Short and curt responses, hardly engaging in conversation, and certainly not laughing around others.
In Sumire's case, what has always stood out to him is the discomfort on her face whenever she speaks to someone.
'She's just a normal frightened girl.' That analysis was so wrong. Almost immediately after they started hanging out with each other. Toh realized there was more to that statement. She isn't just frightened. Her scars run far deeper than any normal wound.
