、。
She looks grumpy, but she's actually kind.
Literal
She [topic-は] grumpy-looking [adverbially-に] looks [but-が], really kind.
気難しい ('hard to please, prickly, grumpy') is a compound adjective — 気 ('mood, spirit') + 難しい ('difficult'), giving 'difficult of mood,' i.e. someone whose moods are hard to manage. 気難しそうに見える uses the seeming-evidential ~そう plus 見える ('to appear, to look') for layered indirectness: she appears to appear grumpy. The clause-final が functions as the contrastive 'but,' setting up the reversal in the second clause. 本当は ('truthfully, actually') is the canonical tag for 'looks-like-X-but-actually-Y' constructions.