Rokuro Okajima is a small-time salaryman who is carrying documents for his company, when the ship he's traveling on is attacked by pirates. Kidnapped, he discovers to his dismay that his employers' main concern is to ensure the documents don't get into the wrong hands, even if it means sending the carrier to the bottom of the sea. Now, with his former life ruined and his kidnappers seeming comparatively friendly, "Rock" decides to join their merry band of mercenaries, and sets out with a new career to the shadier corners of the South China Sea.
Both Black Lagoon and Zero In feature this non-combatant guy who comes up with strategies and plans for his trigger happy gunslinging girl companion (or in case of Zero In girls). In case of Zero In it takes several chapters before the character roles get established. If you like action series with lots of gunfights, you probably can enjoy both of those manga.
Sakura Sakurakouji may seem like a frail and demure girl, but in reality she’s a powerful martial artist with an unmatched sense of justice. One day, on her way home from school, she witnesses a boy murdering several people with bright blue flames. That boy is Rei Ogami, a merciless assassin known as a "Code Breaker", and the latest student to join Sakura’s class! While Rei appears to be the perfect gentleman at school, in truth he lives by the principle of "an eye for an eye" and is hired to target and dispose of criminals who have escaped the law. With everyone oblivious to his true nature, only Sakura knows what he really is and his remorseless feelings towards what he does. But despite his icy heart and his threat to burn Sakura and everyone around her should she reveal his secret, the righteous teen can’t help but feel that there’s more to Rei than meets the eye...
Both are action stories, about police, flavoured with special abilities (though Code:Breaker special is more like unnatural, while Zero in special is just stronger than normal ). Both are about special organisations fighting with most brutal crimes. Their stories differ a lot, but if you like one version of "criminal hunting with shounen manga feel", than you should probably enjoy the other one as well.
If you liked Zero In there is a slight possibility that you might like Dollgun too. Well, Dollgun is far from perfect, has large quantities of pointless ecchi in some of the chapters and there is a harem going on (kind of). However the relationship between the male lead and female lead in Dollgun did remind me of Zero In and that is the real reason of this recommendation. Also, the male lead in Dollgun gets to kick ass and even man up as the story progresses, which in case of Zero In was only limited to manning up.
Both Gunsmith Cats and Zero In are those "girls with guns" (and sometimes with swords or explosives) series. The girls in Gunsmith Cats are bounty hunters, while those in Zero In are police officers. Zero In features those (ridiculous gun-fu like) weird combat skills, on the other hand Gunsmith Cats aims for more realism when it comes to combat, but there is a bit of explicit content there. There are some chances that you might like one of those if you liked the other so, you might as well give it a try.
I don't understand it myself, but somehow Zero In reminded me of Kaibutsu Oujo. Maybe it was because of the semi-episodic nature or because of the fact that the characters in both series often have to investigate something. I can't really tell, but if you liked one of those two manga, you could try reading the other one as well.