What do you think Rio was trying to imply with the “you know me...” he says to Nick?
I talk about that conversation a little bit here and here, but don't dig too deeply into that specific line, so I'll try now!
NICK: You know, I think I finally figured it out.
NICK: Why she's got you by the jewels. [Lick Mick, Nick realizes Beth has a "hold" on Rio and that she has the power to influence his decisions, and he specifically frames it in a way to suggest that Rio is being emasculated by her]
RIO [chuckling]: Oh, yeah? [Rio opts to play nonchalant and amusedinstead of outright denying this]
NICK: Yeah, it's always the spicy ones. [Nick suggests Rio's interest in her is because she's a little "crazy" or "wild." There's innuendo here as if Nick might still be trying to pass off that he and Beth slept together, so he "gets" it from Rio's POV now]
RIO: My kinda girl. [Although there are elements of truth to this quality of Beth's being something Rio is attracted to, it is a sliver of a more complex picture. Still, Rio plays into Nick's idea of who he is and what he likes]
NICK: Yeah, well, makes me nervous. [Nick pivots the conversation]
RIO: Don't be. [Rio denies Nick has any reason to be nervous although he's awkward throughout the entire conversation because Nick has good reason to feel nervous and Rio and Beth's relationship as they are actively plotting against him together]
NICK: Well, 'cause, when it all goes south, I need you to be clear-headed. [Nick alludes to their overarching plan of using Beth for their own needs with her new spot on City Council, and the fact that they're willing to sacrifice her to protect themselves. He's saying he doesn't want Rio's feelings for Beth getting in the way of him throwing her to the wolves when need be. The irony of Nick saying this is that this is the exact reason that Rio and Nick's relationship has finally deteriorated to the point that Rio is ready to betray him]
RIO: No, she's just some bitch, bro. [Like he has done before, Rio pretends Beth is not special to him]
NICK: Oh, she's more than that. [Beat] Hmm? [Laughs] Come on. [Nick tries to get Rio to admit that she's more a notch in Rio's bedpost that Rio's indifferent to]
RIO [laughing]: You know me. [This line is purposefully ambiguous. Rio's already leaning into Nick's conception of him in this conversation, and he wants Nick to fill in the blanks as he likes here. When it's first said, though, there's an implication that Rio means that his usual pattern with women is one of casual attachment]
NICK: Do I? [Nick signifies that he doesn't trust that he does know who Rio is or what he values or cares about anymore—that he's been so influenced by Beth that he's changed]
RIO: I'll do what I gotta do. [Rio shifts the meaning of "You know me" to reiterate that in the end, Rio always does what's best for the business and therefore their partnership. That Nick has nothing to worry about because when push comes to shove, Rio will sacrifice Beth. However, there's verbal irony here because while he means for Nick to interpret it that way, what it really means is that he'll do what he has to do to protect his relationship with Beth—i.e. sacrifice Nick]
Basically, I think Rio was trying to downplay Nick's worry by emphasizing his and Nick's history and familiarity and hoping that it would be enough to dilute Nick's suspicions. The problem, of course, is that while Rio wants to sacrifice Nick to save what he has with Beth, and while he is committed to the plan, it's not really easy for him. In dredging up the ways that Nick does know him, Rio reminds himself of why this betrayal weighs heavily on him at the same time that it signifies how far they've drifted from each other because Nick both did know Rio well enough to know that he'd changed and Beth was a threat to them but he didn't know Rio well enough to know which lines not to cross, ultimately dooming their partnership.