[Once again Shiraishi's faced with the feeling that he should say something. Maybe it's the big brother instinct or something entirely else, but he feels like it would be bad to just say nothing at all.
What to say is something he's not sure about. "I'll try not to disappear." would feel awkward, because he's already done that once and come back without his memories. Even worse, he came back with no memories of Miyuki at all. Besides, it wasn't even their call if they did or not. "I'm sorry" is also something that he'd like to say, but that might make them both feel uneasy. Miyuki didn't need more uneasiness with what she had.
Was he really in any position to say anything at all? Maybe not. He would have still liked to be there to support her somehow. They were from the same world after all and they would know each other in the future.]
1/2 not here | I'm sorry for the tl;dr
What to say is something he's not sure about. "I'll try not to disappear." would feel awkward, because he's already done that once and come back without his memories. Even worse, he came back with no memories of Miyuki at all. Besides, it wasn't even their call if they did or not. "I'm sorry" is also something that he'd like to say, but that might make them both feel uneasy. Miyuki didn't need more uneasiness with what she had.
Was he really in any position to say anything at all? Maybe not. He would have still liked to be there to support her somehow. They were from the same world after all and they would know each other in the future.]