。
She is shy rather than cold.
Literal
She [topic-は] cold [rather than-よりも] rather shy [explanatory-なのです].
よりもむしろ ('rather than X, more like Y') contrasts two possible descriptions. 内気 (うちき, 'shy, introverted') is a na-adjective. The explanatory なのです frames the correction as background information — 'the truth is, she's shy rather than cold.'