Regan is an FBI agent who often has to deal with hijackers at airports. He is a unique character because he is one of the few living people who know about Golgo 13.
Golgo 13 has killed innocent inspectors, officers and agents in the past just for knowing his habits.
Regan is one of the few people who can contact Golgo 13 in emergency situations when his skills are required.
Regan made his debut in episode 1 of the Golgo 13 anime series and was voiced by Rob Mungle in the English dub.
Regan made a return appearance in episode 21 but was voiced by George Manley.
Each Golgo 13 episode is a stand-alone story so it's rare to have continuity. It's possible the voice director forgot about casting this character previously, since the majority of characters in the series appear only once.
It is worth noting that in episode 21 Rob Mungle voiced a different character (Falcao) so maybe the voice director didn't want him voicing multiple characters in the same episode.