The dynamic relationship between Rock and Revy from Black Lagoon is one of these types of relationships. And it exists on many different plains: friends, romantic, criminal, and a struggle between good and evil.
When they found him, the Lagoon Company wanted one thing from Rock, the disc he was carrying full of his companies secrets and they figured they could ransom him for extra cash. But as they slowly realized, the Japanese company they took the disc from to sell to Hotel Moscow (the Russian mob), the ones that should have been concerned about getting Rock back to Japan safely; didn't actually care and were willing to just say he had died in the South Pacific. They treated him like garbage, like he was expendable waste. It was with that sight, seeing someone treated like nothing and like they didn't exist, Revy broke character and invited him to join the Lagoon Company. What you get the sense from Revy's general character, the reasons she may have asked Rock to join is because she saw a sense of normalcy joining the group. Someone "normal" who's lived/was living a "normal" life because that's not something she was used to; ever since she was a kid she'd been stealing and murdering, same went for Dutch, and Benny was always in trouble as well. Rock though, he brought a fresh-air to the group, to her specifically, and I'll get to why exactly she might want someone normal in their group of pirates.
As friends they start out okay until they have an argument in a sunken Nazi U-boat about "right" and "wrong"; thinking that taking medals and other loot from dead Nazis to sell is wrong Rock lets Revy know about that. Revy takes it more personally though and feels like Rock is judging ALL of her life decisions and not just this. She makes it very clear to him that what he thinks is "right" isn't necessarily "right", and that if he ever looks down on her again, that with no hesitations she will kill him. They end up in a massive argument and fight in the marketplace where they make up over the issue. Once Rock explains to her just it is why he's so upset, that he feels like Revy is becoming someone who will do whatever for money even betray others, to be like his old bosses, and that seeing her like that upsets him. Revy immediately just stops fighting because she feels like Rock's compliment towards her, thinking of her as a "saviour" for him is something that just makes her revert to herself. It's also where they share the infamous "cigarette kiss" scene (implying Revy's actual feelings for Rock and how much him looking down on her actually hurt her). This is I think the turning point for the friendship as they become closer. As time goes on, Revy and Rock become inseparable, that their friendship and relationship is what drives the story; Revy will always go to bat for Rock and he's willing to stick up for her as well. It's also where you can see that tinge of romance start to show.
There are moments that you can tell that the romantic spark is there, more-so for Revy's feelings for Rock, but as time rolls on you can see a vice versa start to happen. When Eda asks if her and Rock have had sex in the year since he got to Roanapur, honestly, she deflects the question by just saying, "it's hot" again but she never says no. And judging from the way she showers freely and is constantly half undressed around Rock. I wouldn't be surprised if, at bare minimum, they've had sex. They're never an item explicitly but Revy's constant over-protection of Rock, especially when Eda tries to hit on him for instance, "You might've come fishin' for men, but this one's off limits. I'll kill you."
One thing I've seen elsewhere and agree with is that Revy looks to Rock as her out. Her salvation from the life of crime, murder, and destruction she leads. That his presence will eventually bring her a normal life too, maybe leaving Roanapur together to go to Japan or somewhere else in the world to just start fresh. It's why you see her try to shield him constantly from falling onto her side of the line, why she says to him, "I'll be your gun"; Revy wants to shield Rock from the criminal world he's yet to truly cross over to, by being his gun. To make sure she's the criminal. To make him stay in the business suits she makes fun of and be the same gullible klutz she's always known, maybe at least wear the Hawaiian shirt she bought him. She becomes especially concerned during two moments: when Rock confronts Balalaika and when he does his massive gamble during Roberta's rampage against the CIA. It's honestly one of the few times I've seen Revy that concerned. But Rock mentions to Revy around that time, "if you're my gun, I'm your bullet" that they need each other but even though he's not truly left the "twilight" like people have said that it's possible but with Revy there he'd be fine. I believe she only wants him finding her an out, to live normally, before he becomes to corrupt and criminal-like and before she is too irredeemable for both of them to actually be happy. I'd hazard to guess based on how they around each other later in the series, how they were in Japan together, that it wouldn't be a friendship if they managed to find someway out of the Roanapur life. The biggest hazard is something CIA agents mentioned, the US government still knows what Rebecca Lee has done so her living in secret in Roanapur is what's also keeping her off their radar. It might be very hard for Revy to start over with Rock even if she wants too.
It sounds like quite a toxic relationship Rock and Revy have, but I honestly don't think it is one. It may seem like that but with how much the two actually care about each other once they worked out their initial trust issues in the beginning, I see that all that happened is they got closer and closer together. Revy willing to risk life and limb breaking into prison camps to bail out Rock, to have his back when he makes stupid gambles, to do things that you wouldn't expect that brash and uncaring Revy to do. That Rock is also willing to do the same for Revy, that he was so distraught that she was becoming like the people he hated that he was willing to stare down the barrel of Revy's gun to prove it. Maybe I'm wrong and maybe they are just sickly co-dependent but I think that with Rock being such a sincere and "relatively innocent" dork that a bit of that rubs off on Revy and she becomes a better person around him.
One thing's for sure, Rock and Revy's relationship is complicated, and Black Lagoon is better off because of it.