Rival-chan existed because at some point in 2015, YandereDev was not satisfied with her appearance as the original Osana Najimi. This version of Osana "didn't look tsundere enough", so Osana's old appearance became Rival-chan. Rival-chan was in the game as a placeholder model, as she was not fully implemented, while Osana has a new appearance and retains her tsundere personality.  She no longer exists in the game and is now the character model for Raibaru Fumetsu.
Rival-chan is an extremely short girl, and is currently the shortest character in the game. She has flushed cheeks, a soft smile, orange-amber eyes, and pale skin. Her ginger hair is worn in two slightly curled pigtails. To keep her hair up, she wears ruffled pink and white polka-dotted scrunchies and a purple ribbon. She wears the default female school uniform, unless customized by the player, and black shoes with white loose socks.
She is highlighted red in Yandere Vision.
Rival-chan did not have a programmed routine. In April 2015, YandereDev said that she would walk around, but that was before Osana's original appearance became Rival-chan's. She stands by the closest cherry tree to the left of the fountain in the plaza.
YandereDev calls her "indestructible", as apparently many players have tried to kill her to no avail.
She appears in the spin-off game Yandere Clicker back when she was named Osana Najimi. Her face was used for the "Imouto" option, described as "A kawaii Imouto who will stab Senpai for you". However, she is not Senpai's sister and should not be mistaken as such. She also makes a small cameo in the "April Fools Parody Trailer", where her photos appear on the corkboard. Knives through Rival-chan's head hold the photos up.
She makes another cameo in "It's Beginning to Look a Lot Like Murder", where Rival-chan was hunted down by Ayano Aishi through the woods and eventually killed.
- She is unaffected by any Easter Eggs or the Demonic Ritual.
- She is the only unprogrammed NPC whose uniform will change according to the player's choices.
- Before the November 15th, 2015 Build, she always stood by the first cherry tree by the school entrance. She was moved to the plaza in the November 15th, 2015 Build.
- If the player pressed Space, Rival-chan would cycle through several animations. This feature was removed when Yandere Simulator began to run in Unity2017.
- She makes multiple cameos in the Challenges Menu, where she is shown as the target of every elimination method.
- Rival-chan's 3D model is available on the Unity Asset store, and her model is named Satomi Makise.
- Her hairstyle cannot be loaded by any other NPCs.
- From the September 1st, 2017 Build to the April 2nd, 2018 Build, players can now load students with her hairstyle.
- Rival-chan makes a small cameo in the Gaming Club by appearing on the stealth game, being stalked by Oka Ruto.
- The reason why her hairstyle was added into game files JSON and Pose Mode was because a fan wanted to pose her.
- It has been removed as of the April 2nd, 2018 Build.
- If you press the H key multiple times, Ayano will wear her hairstyle.
- She was accidentally disabled in the July 1st, 2019 build, but was later restored in the July 3rd, 2019 build. She was re-disabled in the July 18th, 2019 build because Raibaru Fumetsu is currently in development.
- On July 15th, 2019, it was confirmed that her design is going to be used for the Mysterious Obstacle, Raibaru Fumetsu. Once the build that features Raibaru and Osana is downloadable, she will be talking, interacting and walking.