Let's revisit the pivotal scene from episode 4x13 for Danny and Rikki! Rikki's pointing Danny's gun at Ollie, but Danny finds them in time and steps in between them, putting himself in the line of fire. When he tells Rikki to blame him for Ruben's death and to shoot him instead, Rikki breaks down and cries and says to Danny that she just wants her son back. Look at Danny's expression as Rikki begins to cry. It's hurting him to see her in pain, and because he's blaming himself for Ruben's death and for causing Rikki this pain, it must hurt a lot. When Ollie badmouthed Rikki, Danny instantly lost it and punched the guy at least twice before Flack dragged him off. That reaction is not of a man who's just distant acquaintances with his neighbor. That's the reation of a man who's pissed off beyond words that some a-hole insulted someone he cared very much for and wasn't going to let it slide. And here's something very interesting I noticed upon watching the scene again: After Flack yanks Danny off Ollie and Flack is telling Ollie to get lost, Danny paces a bit to cool off ... and the first thing he does is head straight for Rikki and hold her arm. What makes it even sweeter is that Rikki does the same thing with him at the same time. Even then, they were already instinctively reaching out for each other. I think this screencap speaks for itself. And yeah, isn't it heartwarming to know Danny called Rikki sweetheart?