Here is what the New York Times is reporting about Mr. Halper:
The informant, an American academic who teaches in Britain [at Cambridge University], made contact late that summer with one campaign adviser, George Papadopoulos, according to people familiar with the matter. He also met repeatedly in the ensuing months with the other aide, Carter Page, who was also under F.B.I. scrutiny for his ties to Russia.
The informant is well known in Washington circles, having served in previous Republican administrations and as a source of information for the C.I.A. in past years, according to one person familiar with the source’s work.
Here is what NBC is reporting:
The professor who met with both Page and Papadopoulos is Stefan Halper, a former official in the Nixon, Ford and Reagan administrations who has been a paid consultant to an internal Pentagon think tank known as the Office of Net Assessment, consulting on Russia and China issues, according to public records.
This is from Wikipedia:
Halper has worked as a senior foreign policy advisor to various think-tanks and research institutions, including the Center for Strategic and International Studies, The Center for the National Interest, where he is a Distinguished Fellow, and The Institute of World Politics where he is a Research Professor. He has served on the Advisory Board of Directors of the Paul H. Nitze School of Advanced International Studies and contributed to various magazines, journals, newspapers and media outlets.
Here is more about Halper's work for the Office of Net Assessment.