Seconded. It's more of a Ranma ½
fic than a Black Lagoon
one, but it's still one of the better fics available and worth looking up to get your fix. Be aware the author is trying to give Gretel a happy ending via Easy Amnesia
, but it's starting to look like said amnesia is being subverted, and the rest of it is worth it. (Also, poor Gretel deserves to be happy, even if it takes an inaccurate trope.) Watching the gangsters' response to things like magic mushrooms or chi attacks is hilarious and those reactions tend to be spot-on in-character; the fights balance the supernatural with the gunfighting quite well, and overall it's worth looking up.