In Sailor Moon, Naru Osaka is introduced as Usagi's "best friend." However, her appearances are few and far between, especially in the manga where she barely shows up after Usagi begins spending more time with the Sailor Senshi. While it's normal for friends to grow apart, Naru seemed to just be thrown to the side and completely forgotten, despite trying to be a good friend to Usagi. Even if she was okay with the split, she at least deserved to know why Usagi wasn't around as much, even if it wasn't the whole truth.
Naru's introduced in Act 1 of the manga and Episode 1 of both anime adaptations. She's shown scolding Usagi for being late for school and eating lunch in the hallway. This sets her up to be a responsible and sensible young woman. She invites Usagi to her mother's jewelry store, as they're having a huge sale, which actually was a trap set up by the Dark Kingdom to steal humans' energy. Naru discovers that her mother has been replaced by a monster and becomes a target, later being rescued by Sailor Moon.
This is where Naru's role in the story splits between the first anime adaptation and manga. She shows up more in the anime, being a target by the villains in nearly every season except Stars. She even falls in love with one of the first season's villains, Nephrite. This romance was short-lived, as Nephrite died trying to protect Naru from monsters sent by Zoisite. She's heartbroken and locks herself in her room, with Usagi coming by and trying to cheer her up. She eventually comes out and goes on to date her friend Umino, a weird geek that hangs around her and Usagi. These relationships were not canon in the manga and Sailor Moon Crystal.
Naru even made an appearance in the live-action adaptation of the series, Pretty Guardian Sailor Moon. This incarnation of Naru was jealous of Usagi's new friends, Ami Mizuno in particular. She later found out about Usagi's alter ego as Sailor Moon and was even injured in a fight, but eventually went on to forgive Usagi for keeping secrets.
Naru hasn't been treated the best in the series, even though she was the first person Sailor Moon rescued. There could have been so much done with her character's place among the Sailor Senshi. Usagi really should have opened up to her about what was really going on after Nephrite's death, but she didn't say anything and just kept Naru in the dark. Even in the DiC English dub of the first anime, Naru wasn't treated with the respect she deserved -- she was renamed Molly Baker and given a thick, grating Brooklyn accent.
Even though Tuxedo Mask is technically the "Sailor Earth" of the group, Naru's proneness to being attacked should have been incentive to make her an honorary Senshi. This would have kept her relatively safe and kept her friendship with Usagi strong, and even help her gain new friends in the Sailor Senshi. Yet, she's told nothing and just continues to be in the dark. We see less and less of her until she's completely forgotten. The last we actually hear of Naru is in a side story where it's revealed she has a younger sister named Naruru.
Naru should have been treated better by Usagi and the story as a whole, especially in the anime. She was attacked numerous times by monsters trying to kill her, watched her first love die trying to protect her from said monsters and her best friend is distancing herself without saying why. It's not uncommon in stories like these for the best friend to find out the secret and even gain powers of their own, but that isn't the case here. Naru is just thrown into danger and forgotten. She deserved so much better.
