Well, in the actual show/manga its more he's his Companies Errand Boy, and while out on assignment Black Lagoon shows up, takes his very important data (Nuclear Weapon tech wasn't it?) for Balalaika, and then Revy takes him (For a ransom)
Things kinda fall into place from there; various obscenities are said, a Hind gets Torpedoed, Rokuro becomes Rock, and the rest is history
Good stuff