Sailor Moon had many changes made during its original 90s run. Mamoru was notoriously made to be in college while with middle-schooler Usagi, storylines were added or changed completely, etc. One of these changes was the addition of Naru and Nephrite's storyline.
Naru was Usagi's best friend before she became Sailor Moon and was the first person Usagi saved after gaining her powers. She made frequent appearances throughout the series, usually being a target for Queen Beryl's minions. One of these underlings was Nephrite, who was brought in after Jadeite was put into Eternal Sleep and targeted Naru specifically to find out Sailor Moon's identity. He initially thought she was Sailor Moon after she figured out that he was sending love letters to various girls under the guise of Tuxedo Mask in a bid to lure Sailor Moon out.
While his plan ultimately failed, the amount of energy he drained from Naru pleased Queen Beryl, so he began manipulating and preying on Naru as entrepreneur Masato Sanjouin. He used her to carry one of his Youma into a party with Princess D from the Diamond Kingdom, which would then possess the princess, with Naru completely oblivious. Eventually, Nephrite came into possession of the Black Crystal -- a tool to help him detect the Silver Crystal -- and was shown Naru.
He thought that this meant Naru was the key to finding the Silver Crystal and asked her to help him find it. As the daughter of a jewelry store owner, she thought one of her mother's jewels was it and took it. She brought this to Nephrite, but the Black Crystal didn't react to it. Instead, it reacted to Naru.
Nephrite was going to kidnap Naru, thinking that she held the Silver Crystal, but was stopped by Sailor Moon, Sailor Mercury and Sailor Mars. During the ensuing battle, Naru protected Nephrite from Sailor Moon's attack, despite all the manipulation he put her through. He would then sneak into Naru's room while she was sleeping and scan her body --which is shown without clothes-- with the Black Crystal, looking for the Silver Crystal. She woke up as he was leaving, and he asked Naru who Sailor Moon was, telling her that he wanted to fight with her rather than against her. What he didn't know was that Zoisite, who had been trying to get rid of Nephrite and hoping he would fail, was spying on him and overheard this.
Zoisite sent three Youma to kidnap Naru to draw out Nephrite. Naru had called Usagi before being taken, which Nephrite listened in on, and his suspicions were confirmed when she transformed in front of him. During their fight, Nephrite sensed that something was wrong with Naru and left, saving her from the Youma, but was injured in the process. They were pursued by the monsters, and Nephrite was injured by an attack that sucked out his life energy. After defending Naru from another attack and Sailor Moon defeating the monsters, Nephrite succumbed to his injuries.
It's clear that Naru's feelings for Nephrite were strong -- strong enough for the Black Crystal to react to them. Usagi knew Nephrite was manipulating Naru and tried to warn her about him, but Naru was too in love and wanted to see the good in him. Even when Nephrite himself said that he might continue to lie to her, she said it was fine as long as he was with her. She never really knew who Nephrite was because he had lied to her the entire time and only knew what he wanted her to. There was also the issue of him being an older man --implied to be at least 19 or older-- while Naru was only 14.
In the manga, it was in fact Sailor Jupiter who defeated Nephrite, and he and Naru never had this interaction at all. Naoko Takeuchi instead had Nephrite and Sailor Jupiter -- or rather, their past lives-- hinted at as being a romantic pairing in an illustration, though she never followed up on this concept. Nephrite would also live on as a gemstone and advise Mamoru alongside the other Shitennou in spirit.
This wasn't the only change made to the main storyline or characters. For example, the last story arc depicted Seiya of the Sailor Starlights romantically interested in Usagi and pursuing her. The anime also made Rei more interested in romance and she briefly dates Mamoru, while her manga counterpart is completely disinterested in men.