I think this is why Kate as a character resonates with me so personally. She isn’t the overexaggerated idea of a wallflower. She’s just the one who doesn’t think anyone is looking because no one would ever have a reason to.
Beyond this, both Anthony and Kate carry with them the unresolved trauma of losing a parent at a critical stage in life — his father and her mother, respectively — and it is working through this trauma as a pair that ultimately brings them closer together
Both are, as mentioned above, the oldest sibling and the one who assumes responsibility for the family. That is a very tough front to put up, and it is one they both employ with stunning regularity and conviction.