Shine's entire backstory. By that point in the comic, the audience already knows she's going to give up on being an idol, so both the lighthearted moments at the beginning where she's goofing off in class and the scene where she naively tells Hasami she dreams of being one are pretty sad given the Foregone Conclusion of what ends up happening to her. Then, her voice slowly starts to malfunction without her having any control of what's happening to her, and she can only plead with her classmates and teacher to believe that she genuinely wants to be an idol and isn't failing on purpose. Watching her get rejected by every single person in her class, including her former friends, is heartbreaking.
The sheer difference in Tech's demeanor between her old self, where she smiled and had a shine in her eyes, compared to her current self, where she's a Perpetual Frowner with Dull Eyes of Unhappiness.
