Sasa is Kosaki's daughter, she was introduced in chapter 229.5 along with Haku Ichijo (Chitoge and Raku's son).
Haru is Kosaki’s sister.
Ruri Miyamoto is Kosaki's best friend. Ruri supports Kosaki in her love-life (a.k.a. Raku) and is almost always the one who always does the first move to put Kosaki and Raku in a situation in which they try to confess their love for each other.
Raku met Kosaki when they were children and at Chapter 221, she was revealed to be the promised girl. They met again when they were both in middle school. Raku considers her to be his ideal woman and is in love with her. While Kosaki is in love with Raku as well, she is too shy to confess to him. Both of them are completely unaware that the feelings they hold for each other are mutual. Kosaki loves Raku because he is kind and caring. Like the other girls who like him, she can be jealous of him spending time with other girls. However, her jealousy never extends to the point of bloodlust like Marika or Chitoge. She often wonders whether Raku and Chitoge's "fake relationship" is actually developing into something more special. Both she and Raku become embarrassed when they say each other's first names, and they quickly revert to what they call each other regularly. Raku wants to spend more time with her but usually cannot because he is under watch by Claude or has to maintain the illusion that he and Chitoge are dating, but when he is given the chance he will take opportunities to go shopping with her or help her at her parents' Japanese candy shop. Kosaki's best friend Ruri, who is aware of the situation, tries to put them into situations that will cause them to admit their feelings for each other. These situations normally fail for random reasons.
It is later revealed that Kosaki is the promised girl. In Chapter 225, Kosaki confesses her love to Raku. Raku earned a flashback of all the things Kosaki did for him and started crying. Kosaki asks him why he's crying which he doesn't know the answer to and both of them started crying. In the next chapter, Raku also says that he likes Kosaki as well, ever since in middle school. She understands however, that someone else is already in his heart. Kosaki gives her key to Raku as he takes it and goes away, thanking Kosaki for what she has done. Ruri then found Kosaki standing all on her own. Kosaki is confused why she isn't crying a lot and she couldn't understand her feelings. She makes a rather happy face as she asks Ruri what kind of face she's making which Ruri simply replies that she looks awful. Raku, who was on his way to find Chitoge, stops and puts Kosaki's key into his pendant. He opens it to find a letter from when he and Kosaki were younger. He finds two rings as well. Kosaki asks at the end of her letter if Raku still likes her, even now. Raku drops onto the ground and tells himself that they've liked each other all this time but they ended up knowing a little too late.
Chitoge shares a friendly relationship with Kosaki, close enough that they call themselves by their first names. She is one of the girls who know that Raku and Chitoge are fake lovers. Chitoge and Kosaki had actually made a promise to each other to help each other out to win over their c that Chitoge and Tsugumi were leaving to America in a week, Kosaki was shell-shocked and sad, finding out that Chitoge would leave (Nisekoi Chapter 161) but when Chitoge comes to her farewell party to tell everyone that her move was canceled, Kosaki rejoices and hugs Chitoge, crying.
When Chitoge leaves for unknown reasons, Kosaki comes to the conclusion that she had run away after she had over heard her confess to an unconscious Raku. She realizes that Chitoge likes Raku as well and decides to go with Raku to New York where Chitoge was stationed. When Kosaki and Raku encounter Chitoge, she is chased by them but is able to hide away from Raku. Chitoge turns a corner believing she had lost both Raku and Kosaki but is surprised to bump into Kosaki. Before Chitoge could run away once more, Kosaki yells out to Chitoge that she likes Raku and that Chitoge shouldn't run away from her feelings despite Chitoge denying having feelings for him.
After Kosaki and Raku return to Japan, they learn Chitoge has arrived to Tengue Plateau, the place where the special promise took place. She and the whole gang arrive at the location and go search for Chitoge. At the course of their search, Kosaki is able to recall all the details of the promise and remember her being the promise girl. Despite learning all these details, Kosaki continues to search for Chitoge hoping that the realization of the promise won't cause Chitoge to easily give up to confess to Raku. She fails to get to Chitoge in time and instead runs into Raku. Kosaki is able to realize Raku's feelings for Chitoge and decides to confess her feelings to him before reuniting with Chitoge who was only several steps away.
After Chitoge and Raku confess their feelings to one another and return to the gang waiting for them, Kosaki sees Chitoge and runs up to her to give her a hug. The two immediately cry.
Years later, Raku and Chitoge are preparing for their wedding. Kosaki happily calls one of them and tell them she is grateful that she is the one making their wedding cake.
Tsugumi is shown to have some affection to Kosaki, but they aren't super close.
Marika Tachibana is one of Kosaki's friends, although they are not shown as very close, they often cared for each other. Marika knew Kosaki liked Raku from the beginning, yet she never tried to confront her. When Tachibana is in trouble with her mother, Kosaki and the rest of the group goes to help her out.