The First Cut Is the Deepest: After Mayuka and Ren break up, it takes about four volumes for Ren to move on, and he only does so after he sees Yuu bring a conclusion to his relationship with Sayuri.
Friendless Background: In middle school, Ren didn't have any friends and was ostracized by the boys for being popular with the girls... and his only best friend lost trust in him after an incident that wasn't even his fault.
Green-Eyed Monster: He briefly gets jealous of Toda when he notices that they get along with each other really well.
Hard Work Hardly Works: When Ren realizes he and Mayuka are beginning to get distant, he does all he can to prioritize her needs. They break up anyway.
Ascended Extra: Sayuri gains more plot significance once the manga begins to deal with her relationship with Daiki. Also, out of Ninako's friends, she is the only one featured on the cover of one of the manga volumes.
The First Cut Is the Deepest: Sayuri begins to avoid Daiki when she thinks he cheated on her on the school trip. She's afraid of talking to him honestly about this because when she was in middle school, her boyfriend, who happened to be Yuu, was stolen by a girl.
Second Love: For Daiki, she is his second love. In regards to herself, Yuutarou may have been her second love, but it's unclear to how many boys she dated before him.
Tsukasa, Tamaki, and Noriko
The Ditz: Tsukasa, who tends to be a blabbermouth.
Shipper on Deck: All of them, including Sayuri, try to get Ninako and Ren together (though they'd do it anyway as long as they're aware of a potential romantic attraction, as they all shipped Ninako with Daiki in the beginning).
Shipper on Deck: Manabu ships Ren and Ninako and has on more than one occasion tried to get them together. It gets to the point where Yuutarou has to call him out and tell him to mind his own business.
Stepford Smiler: Manabu shows a minor case of type A at the end of his side story after Ritsuko, his First Love, goes to study abroad.
I Want My Beloved to Be Happy: He dated Sayuri when they were in middle school. When Sayuri starts believing Daiki could be cheating on her, rather than taking advantage of the rift between their relationship, he encourages her to make up with him.