Dennis Mitchell.
Dennis Mitchell was a Cabinet minister in the government of Henderson Dewey. Sobel doesn't say which Cabinet post he held, although it was likely a senior one. Sobel notes that like Dewey's other appointees, Mitchell was articulate and popular, and like Dewey himself was able to present an appearance of modesty, understatement, and coolness, and appeared to great advantage on vitavision.
After Dewey's unexpected death on 10 May 1929, Collins was apparently not in the running to succeed him as leader of the Liberal Party. It is reasonable to assume that he remained in the Cabinet after his colleague Douglas Watson was named to succeed Dewey as party leader and Governor-General. He may have been one of the three Cabinet ministers who resigned on 31 July 1934 in protest against Watson's military appropriations bill, though Sobel doesn't specifically say so.
Dennis Mitchell does not have an entry in Sobel's index.