to open up (to someone); to become unreserved; to lower one's guard; to become friendly (with); to feel at ease; to take a relaxed attitude (to)
彼は初めは無口だったが、次第に打ち解けた。
He was quiet at first, but gradually opened up.
新しい職場の同僚たちに少しずつ打ち解けている。
I am gradually opening up to my new coworkers.
もともと人見知りな彼が誰とでも打ち解けるのは珍しい。
It is unusual for him, who is naturally shy, to open up to anyone.