Ryu was retired Sen. Chuck Hagel's foreign-policy advisor and interacted with then Sen. Barack Obama during the summer of 2008, when the two senators traveled together to Afghanistan and Iraq.
As an analyst at the State Department, Ryu butted heads with former Undersecretary of State for Arms Control John Bolton over proliferation issues. He then worked for former Deputy Secretary of State Richard Armitage as a special assistant before joining Hagel's staff.
After signing on as an advisor to the Obama-Biden transition, Ryu later assisted Susan Rice in her confirmation hearings to become U.S. ambassador to the United Nations, leading to speculation -- and some preemptive resentment among Democrats -- that he would get a high-level NSC job. The NSC director position has him in the NSC, but on the payroll of another federal agency for now.
Ryu "is a man of immense character and integrity," Hagel told Foreign Policy in a January interview. He has "a good global assessment of reality and policy ... and can talk simply, straightly, directly."
Hagel, who has succeeded General Jones as chairman of the board of the Atlantic Council, credited his close relationship with the national security advisor as well as his time with Obama on the Senate Foreign Relations Committee (SFRC) in helping position his former staffer for the job.
"The connection started to develop when Obama was on the SFRC and Rexon was my foreign-relations counsel," Hagel said. "Rexon got well acquainted with Mark Lippert, who was Senator Obama's foreign policy advisor," and is now the NSC chief of staff.
"I've known Jones for a long time," Hagel said. "He's really one of the premier geopolitical thinkers in our country. I really always admired his thinking and depth and completeness of how he saw the world."
Ryu is not the only former Hagel staffer to switch political sides. Michael Pevzner, Hagel's former liaison on the Senate Select Committee on Intelligence in the 110th Congress, now has the same position for Sen. Evan Bayh (D-IN), replacing Todd Rosenblum, who has become a deputy to the DHS undersecretary for intelligence. Chad Kreikemeier, who was Ryu's deputy, is now Sen. Jeanne Shaheen's (D-NH) foreign-policy advisor as she gets up to speed as a SFRC member.
"For all those conservative Republicans who complained that Hagel was a closet Democrat on foreign policy matters, these staff moves are compelling evidence," a Senate staffer noted.
Ryu and the NSC spokesman did not immediately respond to queries.