Kairi was gilded rope. She was flashy on the outside, shining and blinding and casting light-shadows on the walls. But inside, she was tough and sea-weary and slightly brittle (but only in certain places, like friends and family and big silly dresses).
She tied everyone together, bound them around the waist and didn't let go, even in the worst of storms.
Her strongest knots were around Sora and Riku, because those bonds had snapped before, hitting all of them with a stinging whiplash.
Kairi wouldn't lose them again; she wouldn't let them be hurt like that again. No matter what.