Ranma does not exactly have a romantic relationship with Nabiki, but they do have a relationship. Ranma hardly shows any happy feelings to Nabiki. Nabiki shows kindness to Ranma for his money, or for just reasons. Nabiki's actions towards Ranma typically stems from her desire to profit off of him.
Hiding from Shampoo
In episode 15 where Shampoo first makes her appearance, Ranma was a girl and tried to find hot water to hide from Shampoo. Nabiki had some hot water, but said it was for 10,000 yen. Ranma refused, and Nabiki poured some in the ground. Ranma was desperate, so gave Nabiki the money. Ranma poured the hot water on herself, turning back into a guy. Only, to get slashed by cold water by Ryoga, turning back into a girl. "Did Nabiki put you up to this?" Ranma said to Ryoga.
As a Couple
The most notable like/hate relationship this duo had was episode 110: "Nabiki, Ranma's New Fiancée!". Happosai took one of Akane's bras which of course prompted her to chase him around the house. While Akane was chasing after him, she she ran past Kasumi, but quickly snagged some sauce from the tray and tried to throw it at him, but hits Nabiki, who recently came home. Nabiki didn't mind, because it was Akane shirt that she was wearing, leaving Akane twice as mad. To try to make her feel better, Nabiki offered one of her clothes to her only for Akane to discover that most of clothes in Nabiki's closet are actually hers. Ranma walks in and said that he used a rag towel to clean the dojo and asks if should clean the towel, but Nabiki requests that he should just throw it away. Akane saw that the towel he used was her own hand-made cat towel. Both Ranma and Nabiki said that the 'A.T.' spelled on the cloth suggested she spelled 'cat' wrong, when it was Akane's initials. She attacked him out of frustration. Nabiki talked Ranma into saying sorry. Ranma tried to face Akane on her balcony, with Nabiki right on the balcony too. Akane didn't want to speak to either of them, and broke the balcony with her fist sending them falling. Akane landed a bit hard, then she realized Nabiki couldn't take a fall. Turning to her concern, it turns out Ranma saved her. Nabiki insisted to put her down so that she could leave, and leave Ranma and Akane, alone. Ranma did not want to. This made Akane mad, and say, "Fine, then why don't you make Nabiki your new fiancee, Ranma?". Nabiki smiled, and held onto Ranma's arm, answering for him, "OK.". Ranma shook his head 'no', but it was already settled.
Next morning, while Ranma was working out, Nabiki wiped his face, and called him 'honey' in front of Akane. After, at school, Nabiki was 'playing' with Ranma, seeing if she could get him to give money, by talking about their relationship with drama. Ranma, of course said,"I don't care how old you are, honest!". Nabiki said she'd call it off for 1,000 yen, but Ranma said no. Later, Nabiki said Akane could have him back for 10,000 yen, but Akane said no. Akane started to have ideas of what might happen, so she went to Nabiki again. Nabiki smiled, and said Akane could have him back for 900 yen. But, Akane still said no. Ranma found out about Nabiki scheme, and planned to get her back by arranging a date. Nabiki was confused, so she gave it to Akane, and said Ranma sent the invite to her. Akane actually accepted.
Next morning, Akane was all dressed for her 'date' with Ranma. Meanwhile, Ranma got roses for the date. Nabiki overheard Ranma's plan, then passed by. Ranma spotted her, and jumped on her bike. Nabiki got him off by saying there was 100 yen on the floor, saw him bash into a tree branch, sending him off. Nabiki smiled, while Ranma said, "I HATE YOU, NABIKI!", snapped the branch in two, and followed her. Akane was already at the maze (the destination Ranma set for the date) and Nabiki saw her. Ranma could hear Nabiki's voice coming from the other side of the hedge, and pulled Akane's arm. Ranma thought it was Nabiki's he was pulling, even though Nabiki said it was Akane. But, Ranma pulled Akane out and hugged her saying, I LOVE YOU, I LOVE YOU, I LOVE YOU, I LOVE YOU!!!". Both Akane and Nabiki had surprised faces. Ranma continued, "Can't you understand that I can't ever let you go? We'll be together for all eternity.". Ranma pulled away from what he thought was Nabiki, but was Akane. He started to freak out, seeing that his plan was ruined. Nabiki let Ranma and Akane's engagement get back together, then left, thinking, That was the stupidest revenge I ever seen!
The manga only arc Playing House focuses on Ranma and Nabiki interaction. While chasing Happosai around the house, Ranma accidentally burns Nabiki's expensive concert tickets. Feeling he is at fault, he decides to stay with Nabiki to repay the debt while the rest of the family goes out .
Nabiki takes full advantage of her situation and exploits him by taking pictures of his girl-form in lingerie, eating expensive sushi, and having him help her package handkerchiefs with Ranma's naked pictures on them. Ranma, feeling guilty, reluctantly agree to all these things.
Ranma then realizes that Nabiki was selling the handkerchiefs worldwide and hits the table, knocking the ink bottle in her direction. Not wanting to pay her back even more, Ranma quickly gets in front of her so the ink lands on him while they both fall to the floor. It's only then that he saw that Nabiki had taken a picture of them on the floor. The picture appeared to show Ranma trying to embrace Nabiki on the floor, and Nabiki decides to use the picture to further take advantage of Ranma.
Ranma then tries to find the picture, but instead finds what he first thought was a love letter to Tatewaki Kuno. However, once opened, it is revealed that the letter was actually a bill for girl-type Ranma's pictures.
Nabiki always seems to have everything planned out for Ranma. When he finds and tears the picture while Nabiki pleads in tears, he discovers she was recording the entire conversation, which sounds like Ranma was attacking Nabiki.
While Ranma was busying himself trying to get rid of the tape and any evidence against him, Nabiki takes final advantage of her situation by stripping in the other room. The rest of the family arrive at this time, and Ranma bargains with Nabiki to put her clothes back on. They finally agree on 3000 yen and Nabiki takes her clothes. When Akane discovers the two, Nabiki nonchalantly tells Akane that she was modeling for Ranma. Akane accepts this explanation.
The rest of the family is oblivious to what transpired in the house while they were absent and the exhausted Ranma tells Akane that Nabiki must have gotten better.
Ranma and Nabiki don't get along too well, but they sure count on each other through thick and thin (like in movie 2, Nihao, My Concubine).