"Stand by the door," Naoto tells him—and so he does.
The victim foresaw his death, requested Naoto by name. Yousuke watches Naoto work the crime scene and can see why: there's something in Naoto's eyes, something that sparked as he entered the room...
Desk, knife, blood and dust. Naoto studies it all, and Yousuke can sense the puzzle coming together around him even if he can't see the picture himself. He tries to understand it anyway, if only in his own head, but of course it's useless.
He's no detective.
That light, in Naoto's eyes—
He doesn't have that.