Once, he was too late. But once was too much. Chat Noir dove through the window, his suit protecting him from the cloud of glittering glass shards that surrounded him, to see the girl - his friend, even now - collapse against the wall as the villain laughed. Chloé's eyes were wide, full of fear and pain, as he crouched beside her. There was nothing he could do. Chat returned to the fight with her blood on his hands and in his mind.
Ladybug's cure brought her back with a gasp and a sob... Brought her back physically, at least. Adrien never saw the light come back to her eyes, the gleam that betrayed the feelings she tried so hard to hide behind layers of snide comments and cruelty. Her spirit was broken, shattered, scattered across the white carpet even after the splinters of glass and scarlet spots were whisked away. She became a ghost, silent and fearful, her flame dimmed to mere embers.
Fire can bite and burn, even kill - but her newly cold soul haunted him. He had failed her.
Adrien's nightmares whispered to him, through the night and then past dawn, a constant murmur of what-ifs and maybes and too-lates, twining together to form a single voice that moaned and sobbed, "who next?"