What is the role of a public prosecutor in criminal cases?

The importance, to the public as well as to individuals suspected or accused of crimes, that these discretionary functions be exercised "with the highest degree of integrity and impartiality, and with the appearance thereof" (People v. Superior Court (Greer), supra, 19 Cal.3d at p. 267, 137 Cal.Rptr. 476, 561 P.2d 1164) cannot easily be overstated. The public prosecutor " 'is the representative not of any ordinary party to a controversy, but of a sovereignty whose obligation to govern impartially is as compelling as its obligation to govern at all; and whose interest, therefore, in a criminal prosecution is not that it shall win a case, but that justice shall be done. As such, he is in a peculiar and very definite sense the servant of the law, the twofold aim of which is that guilt shall not escape or innocence suffer.' " (Id. at p. 266, 137 Cal.Rptr. 476, 561 P.2d 1164, quoting Berger v. United States (1935) 295 U.S. 78, 88, 55 S.Ct. 629, 633, 79 L.Ed. 1314.)