Osana Najimi

'''Osana Najimi is the first officially introduced rival for Senpai's affections and will be used as a tutorial at the beginning of the game. She is a student at Akademi High. She will be implemented within several months according to Druelbozo. YandereDev first needs dozens of male and female animations for Osana to be implemented. '''

It should be noted that the character who stands under The Cherry Tree southwest of the plaza is Rival-chan. The original design for Osana had Rival-chan's adorable appearance, but YandereDev thought she didn't look tsundere enough. There is now a new appearance for Osana. Rival-chan is the person with the old model, who is now considered a different character.

Osana's original model was not active because YandereDev wanted to give her a different character model to make her more unique, but it was not compatible with all of the animations that were made for the female skeleton that the rest of the girls have. She couldn't be properly implemented unless a character rigger made a control rig for her and an animator used that rig to make a full set of animations for her, or YandereDev gave her the same character model and animation as all of the other female students.

The downside of giving her the same character model as all the other female students is that she will look like a clone of Yandere-chan, but with different eyes, hair, and stockings. YandereDev could not find anyone to rig or animate her original model, so it is possible that this will be the case. Even with the old model out of the way, it may be a while until Osana's new model is implemented, because YandereDev does not want her added without also putting in a whole bunch of Osana-specific "event" animations. Druelbozo, a volunteer, has helped with her model, so she may be implemented within several months.

Original
Osana's original look had hazel eyes and her hair in two golden orange pigtails. Her hair was tied up in two plump scrunchies and she had a purple bow on her head. She wore white leg warmers. This appearance was given to Rival-chan, a stand-in for Osana.

Current
Osana-chan's new model has long, orange twintails that reach her thighs. The ends of her hair have a small yellow gradient. Her hair is tied up in thin pink scrunchies with white polka dots, and she wears salmon pink socks with white polka dots. Her shoes will be black. She wears a small red bow on her head. A quick glance of her back was shown in, "Dumpsters, Buckets, Bathing, and Electrocution in Yandere Simulator". The video "Past, Present and Future" shows also that this appearance has been confirmed.

Introductions
In the original introduction of the game, after Yandere-chan bumps into Senpai, she can be seen talking to him, acting like a stereotypical tsundere. You can see the first intro on YandereDev's YouTube channel., but this is now very outdated.

In the second intro added in the August 15th, 2015 Build, Osana is mentioned by Info-chan, as she requests for Yandere-chan to murder the girl. Info-chan expresses that she wants Yandere-chan to "make her suffer". It is currently unknown what grudge Info-chan holds against Osana.

Other
Osana had a small cameo in one of YandereDev's videos, "I Want My Senpai Back". She was the main antagonist of the video, and stole Yandere-chan's Senpai. She was killed off at the end of the video by being stabbed to death, possibly around 37 times.

Osana has another cameo in another one of YandereDev's videos, "It's Beginning To Look A Lot Like Murder - A Yandere Simulator Christmas Carol" She appears briefly for the lines, "Senpai's bothersome childhood friend is flirting with him again... She wants him, but I'll never let him go!", "A shiny sharp axe and one glorious whack to eliminate Osana-chan!", and possibly, "Lots of blood and gore!"

Personality
Osana Najimi is a tsundere, which is a person that is very sweet and kind, but projects a hostile attitude, often only towards their love interest. It's not very likely that Senpai would know about her crush on him, as she repeatedly insults him and he seems oblivious. Regardless of Osana's rudeness and Senpai's ignorance, Osana is still very considerate. In the original intro, she is shown walking Senpai to school.

The second intro gives no indication of her personality, but "I Want My Senpai Back" makes her look like a tsundere, so she is likely still a tsundere.

She may not be a good friend to Senpai, considering that if you get rid of her non-lethally and Senpai's sister dies, she will not go back and comfort him.

Senpai
They were childhood friends in the first intro. Osana has a crush on him, although he does not seem to be aware of it. The first opening shows Osana and Senpai together, and even walking to school with each other. Osana yells at Senpai, but they still seem close.

There is no information about their relationship in the second intro. Osana still has a crush on him, though, and will still confess her love to him on Friday. However, if Senpai's sister dies then Osana, if eliminated peacefully, will neglect him, so she probably doesn't care that much about him.

Yandere-chan
Osana Najimi is a target of Yandere-chan because she is a rival for Senpai's affections. In the first intro, Yandere-chan has qualms about killing others. She simply wants a love life and Osana is in the way, but Info-chan, a glory-hungry journalist, gives Yandere-chan the idea to kill Osana.

In the second intro, Yandere-chan already wants Osana out of the way by any means necessary. Info-chan just encourages her more and tells her about Osana's plan to confess to Senpai.

In the "It's Beginning To Look A Lot Like Murder" video, Yandere-chan mentions that Osana is "flirting" with Senpai "again". This may indicate that Osana's tsundere demeanor is a way of flirting with Senpai, but it's also possible that this is just how Yandere-chan interprets her actions.

Info-chan
In the first opening, Info-chan didn't hold anything truly personal against Osana. However, she was once the main antagonist and was going to be a rival for Senpai. The original intro also shows that Info-chan wanted some terrible events to unfold at Akademi High to boost her newspaper sales. Her motive for wanting Osana dead might have been one of the two, or both reasons, depending on when Info-chan being the main antagonist was changed.

The second intro does not state why Info-chan is going against Osana. Info-chan simply wants Osana to suffer, and anything about being a journalist is never stated. There isn't even a Newspaper Club at Akademi High anymore, which makes it unlikely that her motive is wanting her articles to be popular.

Trivia

 * Osana Najimi's name is a pun on osananajimi (幼なじみ), a common trope in the romance genre which means "childhood friend" in English.
 * In the final game, Osana may tease Senpai about how he stands under the cherry blossoms all day, a reference to how long Rival-chan was under the cherry tree.
 * YandereDev said on Twitter that Osana's birthday will be in January, most likely the 1st.
 * Osana will perform the same "hip sway" animation as "Sansdere-chan" from Bad Time Mode.
 * She is the second tsundere character to be introduced by YandereDev, the first being Tsundere-chan from Kuudere Simulator. YandereDev has said that Osana Najimi is "Tsundere-chan with an actual design that had effort put into it."
 * At one point, YandereDev considered splitting Osana into two characters--a gentle childhood friend with Rival-chan's appearance, and a tsundere childhood friend. They wouldn't have been sisters, however, as that would mean eliminating one sister might affect the other so strongly that she couldn't consider dating.
 * In the video "I Want My Senpai Back", Osana is voiced by Brittany Lauda, who also plays Audrey from the popular adult game HuniePop. They both have the same tsundere personality.
 * Osana's design was inspired by anime characters such as: Asuka Langley Soryu from Neon Genesis Evangelion, Rin from fate, and her twintails and stockings were inspired by Michiru from Grisaia.