What are the nature of courts martial? Do they belong to the judicial branch of the government?

Courts martial are agencies of executive character, and one of the authorities "for the ordering of courts martial has been held to be attached to the constitutional functions of the President as Commander in Chief, independently of legislation." Unlike courts of law, they are not a portion of the judiciary. 

"Not belonging to the judicial branch of the government, it follows that courts-martial must pertain to the executive department; and they are in fact simply instrumentalities of the executive power, provided by Congress for the President as Commander in Chief, to aid him in properly commanding the army and navy and enforcing discipline therein, and utilized under his orders or those of his authorized military representatives." "It must never be lost sight of that the only legitimate object of military tribunals is to aid the Crown to maintain the discipline and government of the Army. (Ruffy vs. Chief of Staff, G.R. No. L-533, August 20, 1946)