Ruby Red

RubyRed is the femslash ship between Ruby Hoshino and Kana Arima from the Oshi no Ko fandom.

Canon
The two meet for the first time as kids, when Aqua is casted in the same movie as Kana. Ruby recognizes the other immediately, since she's already well-known as a child genius. However, they don't take a liking on each other; Ruby feels extremally repulsed and annoyed by Kana's big ego and the way she is "fake". After Kana makes a mean comment about Ai's acting, she gets really mad, although she tries to keep it cool. Later, Ruby is watching the shooting of the movie and she witnesses Kana breaking down into tears due to considering Aqua's performance to be a way better than hers. She cries and begs the director for retake, but he refuses to do that, since he doesn't see any problem.

Years later, Hoshino siblings apply to Yoto High School. While they're chatting on the corridor, Ruby mentions Aqua's name and Kana, who happens to walk by, hears it and with excitements starts to talking to the boy, who doesn't recognize her at first. However, Ruby, just as when they first met immediately says "Ah, isn't she the genius child who will lick a lying snitch?" and Kana angrily corrects her that it's "flick a crying snitch"- this conversation is almost the same, as the one they had years ago. When Kana is trying to process the fact that Ruby is the one applying for performing arts programmed, the girls whispers to her brother that she never really liked her, what makes Kana mad. After that, Ruby teasingly calls Kana "Loli-senpai" and offers her to be friends.

Ruby learns that Kana managed to convince Aqua to play with her in the adaptation of "Sweet Today". As they watch the episodes that have already been realised, Ruby, who's a fan of the manga gets really disappointment, calls it atrocious and even mentions, that Kana's acting used to be a way better. When the other girl hears that from Aqua, she shouts "Shut the hell up!" and comments that if Ruby says that, she can just jump off the cliff. Then, she admits to the fact that for the sake of the show, she is holding back and purposefully putting on a bad performence, because it's the only way for her to not be left behind. She puts the quality of the production as a whole over her own repurtation.

Kana spots Ruby and her brother on Yoto High School Entrance Ceremony and congratulates them. After that, she shows them around, assures Ruby that there's nothing to worry about and welcomes them in the showbizsnes.

Ruby is desperate to become an idol, but they can't find any other potential members. When Aqua suggests Kana, Ruby realizes that, based on her own experience as an idol otaku, the other girl is exactly the type of person that fans would go crazy about. Yet, she is too afraid to invite her, because she considers their relationship to be too unusal and she's afraid that Kana dislikes her. Aqua then messages Kana that he has something important to tell her and asks her to meet him after school. She gets angry, when Ruby welcomes her with "You've kept me waiting" and the moment she learns the person that wants to talk with her isn't Aqua, but his sister, her attiude changes and the atmposphere between girls is tense and unpleasant. Finally, Ruby asks Kana if she would join her idol group, what takes the other girl by surprise. She is afraid that it would be a terrible choice that would ruin her career, yet, she also notices that Ruby reminds her of Ai, an "ideal idol" and recognizes her potential. When Kana's about to turn Ruby's offer down, because of the risk and that fact she think of herself as "not cute enough", Aqua manages to convince her and Kana, still being full of doubts and screaming that it's a bad decision, signs the contract with the Strawberry Productions. Ruby then cheerfuly says "Let's give it our best, senpai!"

Fanon
The ship isn't really popular, especially since Kana canonically has a big crush on Aqua, but more people start to acknowledge it.