Si-eun always spoke in a monotone voice. It was just how he was. That didn't mean he didn't feel anything, nor did it mean that he was sad all of the time. It was just how he talked normally, occasionally throwing in a hint of emotion to his voice in order to help get his point across.
His classmates were used to his often uninterested sounding way of speaking, even though his classmates would stab Si-eun in the back; the shorty male did care about what they were saying, even if their responses were in a negative way— He cares. Just that he's very good at not showing it.
Then he's snapped out from his deep thought when he heard heavyweight foosteps behind him, so he turned his head around to look over his shoulder and saw you, whose seemingly taking the same street as he was.