Fetching details from the dictionary...
hard to please; difficult; particular; fussy; choosy; picky; fastidious
bad-tempered; in a bad mood; displeased; sullen; crabby; cranky; grumpy
あの教授はとても気難しい。
That professor is very hard to please.
顧客は気難しかったから、何度も直した。
The client was so picky that I revised it many times.
彼は気難しく見えるが、本当は優しい人だ。
He may seem difficult, but he's actually a kind person.