Kotetsu glowing catches him off guard, too. He doesn't entirely realized it's happened until it's happening, until he's watching Frisk go flying through a haze of shattered, frozen flower petals tumbling down between them. He gasps, horrified, dropping his toy for the sake of covering his mouth. Frisk hits the ground and for a long moment Kotetsu doesn't breathe, until at last they sit up and he feels like his heart starts beating again.

"Oh god, I'm so sorry! Are you okay?" His vision is still tinted blue and he can feel himself starting to cry, frustration and guilt both impossible to hold back. "I'm sorry! I didn't mean to! Are you okay, please be okay, p-please don't be mad...!"

He's torn between going closer to see if they're okay and staying well back, like he should have done all along, and so the end result is Kotetsu fidgeting in agitation on the spot.