He secretly had feelings for her.
密かな楽しみとして、夜中に小説を書いている。
As a private pleasure, I write novels late at night.
They were secretly planning a new business venture.
She had a secret ambition.
He was secretly thinking of her.
The coach was secretly placing his hopes in the young player's talent.
こっそり is a casual adverb emphasizing sneaky, furtive action, while 密か is a more literary na-adjective that can describe hidden feelings as well as actions.
密か is a common na-adjective meaning 'quietly/secretly' for personal feelings or small acts, while 隠密 has a more formal, operational nuance.
公然 means open and public, the direct opposite of 密か, which describes something done secretly or hidden from others.
inwardly; secretly; in one's heart
Rarely used kanji form.
Rarely used kanji form.
Search-only kanji form; not used in modern writing.