Rivals are the main antagonists of Yandere Simulator. In the full game, there will be ten rivals for the ten weeks of Story Mode, with one rival appearing each week. Only Osana appears in the demo build of the game, but they have all been introduced in a video.
They're the most important people to eliminate, as they all have a crush on Senpai (with the exception of Senpai's sister). If the rival is not eliminated by the end of their week, they will confess their love to Senpai (or make him promise to never have a significant other, in the case of Senpai's sister) under the Confession Tree behind the school on Friday, and the player will get a Game Over. A girl confessing to a boy underneath a cherry tree is a cliché in anime and will be drawn upon in the game. Each rival in the game is an anime archetype, inspired by popular concepts and stereotypes. However, not all of them will be based on a "dere" archetype.
In the debug builds, there were no actual NPCs who function like proper rivals, so there was no way to win the game yet.
In the full game, they will not be in the school until their week comes. Each rival will be introduced via a cut-scene conversation with Info-chan. The rivals from weeks two to ten will each have a reason for not coming to the school for the previous weeks of the game. They will also have a short cut-scene that shows how they met Senpai and why they are attracted to him. Some of the cut-scenes will be humorous to balance out the other dark elements of the game.
Senpai will not fall in love with girls of his own accord. He will start to develop a crush on a rival if she has five interaction events with him. There will be one "event" with Senpai each day, with the final event being a love confession. It is possible for Ayano to sabotage each event.
Even though the main object of their desire is Senpai, all rivals will have a secondary crush on another male student. This allows the main character to match every rival with their suitor as a way of eliminating them. If the player chooses to make Senpai female, then all of the rivals will be directly bisexual and/or pansexual, and Senpai will be homosexual, bisexual or pansexual.
Every rival wears special stockings to make themselves stand out and will have unique panties that suit their personality. In the future, rival hairstyles and accessories might be unlocked after beating the game once.YandereDev has considered the idea of allowing the player to dress up as any of the rivals after eliminating them.
As the game progresses, each rival is more difficult to defeat than the previous one, sometimes because of popularity.
Rivals are seen outlined in red in Yandere Vision, although this outline might have other identifications, as traumatized students also have a red aura. These rivals will have a "Red String Of Fate" connected to Senpai.
The rivals will be able to realize that the player is stalking them, and react accordingly.
However, they will never know that they have competition for Senpai.
NOTE: These rivals are only for testing and are not rivals in the final game. Currently, there have only been three test rivals.
Rival-chan uses the original model for Osana Najimi, so they are often confused with each other. Rival-chan only existed to test the red rival outline of Yandere Vision and to stand in for Osana. She now shares the same appearance as Raibaru, Osana's friend and constant companion.
Kokona Haruka was the second test rival. Unlike Victim-chan, she was not removed from the game, and is now a normal student with no crush.
Story Mode Rivals
This section contains spoilers.You have been warned!
All rivals have been planned out with official rival designs since February 2016. Back in late November 2015, an artist had created full-body illustrations for every rival character. YandereDev asked for a few modifications, and then promised that he would officially reveal all of them in his upcoming Rival Introduction Video.
Only Osana Najimi, Amai Odayaka, and Megami Saikou currently exist in the game. Osana is fully functional. Amai Odayaka makes an appearance in the demo on Saturday and during the Amai Challenge. Kizana Sunobu could've been added with the json files, just like Osana used to, but was quickly removed due to the fact that she was only added for a video. Oka Ruto was in the game for a decent amount of time before getting pulled for the development of her as a rival. Muja Kina was in the game as the nurse before getting replaced by the official nurse, because Muja is only a substitute nurse. Megami Saikou can be seen on the laptop, if Ayano kills a student council member.
As of 28th of September, 2016, the rivals have been revealed in the Rival Introduction Video.
Osana Najimi is the first rival in the full game, and is in the demo. She's a tsundere and Senpai's childhood friend. They walk to school and walk home everyday, since they live very close to each other. If Osana is eliminated, it will have a bigger effect on Senpai's sanity compared to anyone else. Her best friend, who is always with her, is Raibaru Fumetsu. She acts like a personal bodyguard to Osana, which will make eliminating her very difficult.
YandereDev has mentioned that Osana essentially functions as a tutorial rival.
Amai Odayaka, the president of the Cooking Club, is the second rival. She is in the Amai Challenge in the demo. She is a warm, gentle, kind-hearted girl. She is a "big sister" or "onee-chan" type character. YandereDev has revealed that she can be eliminated with the "Drive to Murder" elimination.
Kizana Sunobu, the president of the Drama Club, is the third rival in the full game. She is one of the popular girls in school that thinks she is superior to others and like royalty. She dreams to be an actress in the future. Since she is a popular girl in school most people will be following her so it would be a bit more difficult to eliminate her. 
Oka Ruto, the president of the Occult Club, is the fourth rival in the full game. She is considered a cross between the Hex Maniac from Pokemon: X and Y, Tharja from Fire Emblem: Awakening, and Tomoko from WataMote. She is also an outcast and considered creepy by many.
Muja Kina, the substitute nurse, is the sixth rival. Her character trait is unintentional sexual innuendos. She is pure and innocent.Nasu Kankoshi will be on vacation during her week as a rival. She will disappear from the school after her week is over, even if she is eliminated peacefully.
Mida Rana, the substitute teacher, is the seventh rival. The teacher's trait is intentional sexual innuendos, and she will be sultry and seductive. She can be called upon by a Teacher's Pet if they discover a corpse or witness a murder. Ayano will be able to match her with a student. She will disappear from the school after her week is over, even if she is eliminated peacefully.
Osoro Shidesu, the leader of the delinquents, will be the eighth rival. Although she will not be evil, it is known that she has been suspended from school for something gravely serious. Not only that, but she is the strongest and most dangerous girl in school.
She is the leader of a gang, and she always has her cronies nearby.
Hanako Yamada, Senpai's sister, is the ninth rival in the full game. Her appearance has been updated as of November 28th, 2015. She does not lust sexually after her brother, but she loves him so much that she does not want him to get a girlfriend, as she wants all of his attention. She is said to be one year younger than Senpai and two school years behind him.
Megami Saikou, the student council president, is the tenth rival in the full game. She will be a lot like Nayuta from Yumina the Ethereal VN. She will also be a strict disciplinarian. She will be the most popular girl in school. She is also going to be the hardest rival to eliminate, due to a number of reasons, such as being impossible to beat in a fight (invincible self-defense) and being near the Student Council.
Like Story Mode, there will be ten rivals, with one appearing each week. They will be created out of pre-existing assets instead of original designs.
The rivals in 1980s Mode will have a crush on Jokichi Yudasei. The player character is Ryoba Aishi, Ayano's mother, and the yandere from the Mysterious and Basement Tapes. Jokichi Yudasei is also Ryoba's senpai.
In Endless Mode, the game will randomly generate a rival each week for an unlimited number of weeks until Ayano gets caught. The player will have to stalk Senpai until they identify the newest rival. They will not have special stockings. They will have the normal functions of a rival, and the player will be able to eliminate them the same way as any rival in Story Mode.
Possible DLC Rivals
YandereDev has stated that he might consider creating DLC rivals in 2014. These rivals would either be non-canon, or would require unique coding to be implemented. If DLC rivals do get added to the game, they would most likely appear after Megami's week.
According to YandereDev in 2017, he's still considering this idea, including Kokona as a DLC rival.
There will be 17 elimination methods that will work on all 10 rivals. Each rival will also have at least one unique elimination method that is exclusive to them. In fact, it is possible that in the full game some methods will be tied to scripted events which only involve the rival. The rivals were built around their elimination methods.
Once a rival has been eliminated non-lethally, they become a regular NPC and all "normal NPC interactions" become available to them.
The player will not be able to threaten or intimidate rivals, only directly eliminate them one way or another. Blackmail will not be available either because the game is about taking direct action against Ayano's rivals.
At one point, there was an elimination method that was similar to tampering with the school diving boards so that the rival would smash their face into the ground, but it was criticized for being "too Looney Tunes".
If the player kills a rival early, the player has an option to skip forward to the next week, or play through the current week.
There is an old post detailing all ten of the rivals. Although outdated, some information has persisted and remained in the game's plan, such as Osana Najimi and Mida Rana.
There is a page that shows most likely outdated designs of a few of the rivals. The artist mentions that they are subject to heavy change, as well. YandereDev linked to the page in older comments on here.
A car bomb would be an appropriate way to eliminate one particular rival, but this is not very yandere-like.
Certain rivals may be capable of self-defense, but Osana Najimi is not be one of these, since she is a "tutorial" rival. Some rivals are the type of people who would be able to catch a knife if it was thrown at them, even if their back was turned. Muja and Mida, however, are not.
Only one or two rivals will ever directly confront Ayano, and they, including a third rival who can defend herself, will fight in manners completely different from one another.
Koharu Hinata had a crush on Senpai in the September 18th, 2015 Build, and was, therefore, a rival. This mistake was changed with the October 8th, 2015 Update.
All of the rivals will be on good terms with Senpai. If he sees them covered in blood, he will not assume they committed a crime. But, if one of the rivals is arrested, Senpai will be in disbelief, and will think that is what they were really like.
It will be impossible to get rid of one rival to instantly cause the elimination of another.
Each rival's birthday will fall on a different month of the year.
YandereDev later revealed that he only used those names because he thought they were common, generic Japanese names.
YandereDev also revealed he wants names that reference who the character is. For example, Senpai's childhood friend is named Osana Najimi because osananajimi (幼なじみ) means "childhood friend" in Japanese.
There was originally going to be a short loli rival, but YandereDev is opposed to the idea now because of all the work and problems it would create.
If the rivals were in Hogwarts, most of them would be Gryffindor.
Only rivals will have dark secrets, not regular NPCs, with the exception of certain characters like The Journalist.
Pregnancy is a possible dark secret of one of the rivals.
If the rivals have siblings, they do not attend Akademi High School. The only exception to this rule is Hanako Yamada.
Some of the rivals are intended to be likable people who do not deserve to die, possibly making the player feel some guilt for whatever they do to them.
YandereDev's current plan as of January 14th, 2016 does not include a rival who Info-chan will refuse to help kill or torture.
Implementing the ability to disguise Ayano as a rival is very tricky, so it cannot be guaranteed.
YandereDev is considering an old idea of taking trophies from rivals, but it is a low priority.
YandereDev likes the idea of breaking into a rival's home and smashing her possessions, but this would only happen if a small town is added.
Killing a rival who is also a club leader will not shut down the club; each club that contains a rival in it will have one more person in it than usual so that it can stay open even though someone died. A substitute will take the place of the normal club leader.
Each rival's personality will be demonstrated through unique animations that other NPCs do not possess. Similarly, they will also react to Ayano's presence in ways that other NPCs do not.
In one or two cases the player will be able to use a former rival as a "tool" to help eliminate a future rival. However, this won't be the case with all rivals.
Befriending a rival may supply Ayano with a permanent reputation gain of +1, per befriended rival.
A befriended, former rival may comment on Ayano not interacting with Senpai.
Although most rivals travel to school through walking or taking a train into town, Megami will arrive by helicopter.
YandereDev is considering the idea of having a rival's ghost appear where they were killed.
Some of the rivals inherited their interests from their parents.
Although Osana is the first rival, Oka Ruto was the first rival to be implemented in Yandere Simulator and is not a test rival.