‘You don’t know her,’ Rosie said, her voice hard. ‘I can tell you think you do. But you don’t. You only see her when she’s putting on a face.’
She's also both right and wrong here. Caddy doesn't see the Suzanne that Rosie sees, but that's not necessarily a bad thing, and it doesn't mean it's not real. I think of it a little like, Rosie sees Suzanne as she is, and Caddy sees her as who she wants to be. Both are important, and it's why the friendship between all three girls matters so much. The story wouldn't work with just two.