Flannagan is a CIA agent and like Regan and Hume sometimes hires Golgo 13 for secret assignments for the government.
Flannagan knows Golgo 13 is a valuable asset and he has no problem helping Golgo 13 avoid trouble from the law.
In the English dub of the Golgo 13 anime series Flannagan is voiced by Todd Waite in episode 1.
Steven Foster (English dub voice director) has told me that for episode 21 Todd Waite was away shooting a movie so for this episode there was a recast. Flannagan's name wasn't listed in the English cast credits for episode 21 so for now the clip will be credited to an Unknown Voice Actor. In episode 31 Flannagan was voiced by Jay Hickman.