Rune is suspicious of Seth, and I assume he senses something the others don't. When you get the aero-prism, and you press "talk", Seth asks to see the prism, which Chaz carelessly hands to him before Rune has to intervene, yelling, "Stop! Chaz, don't!" Rune apologizes to Seth explaining to him that they cannot be merely curious with the aero-prism, or something along those lines.
I also like the scene where Chaz shows off a bit towards Seth, telling him that as a result of years of training, that is why he is so strong, to which Rika asks, "When did you do years of training, Chaz?"