Invisible doesn't mean it doesn't exist. The woman behind him was vaguely visible in his changed gaze, stiff smile, and perfunctory kiss, her outline highlighted. When love is diluted by time, all efforts to snatch it are in vain. When he told her about his relationship with the invisible woman through his lips, she calmly accepted the inevitable outcome of the breakup. She understands that if love is not there and she tries so hard to save it, she will end up with an empty skin with a shell but no heart. She didn't want the heart-breaking coldness, she was afraid of the coldness creeping into her skin through her clenched hands.