He's sometimes bad-tempered but he's a good fellow ______.
at heart (= used to say what sb is really like even though they seem to be sth different): theo cách cơ bản nhất. "He' sometiems bad-tempered but he's a good fellow at heart." (Thỉnh thoảng anh ấy nóng nảy nhưng về cơ bản, anh ấy là người tốt.)