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Sasa is Kosaki's daughter, she was introduced in chapter 229.5 along with Haku Ichijo (Chitoge and Raku's son).
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 Ichijo is Kosaki's previous love interest. Both of them were said to be childhood playmates along with Chitoge and Marika. Although their feelings were mutual, they were unaware of it. There are multiple times where they try to confess their feelings but there had always been something that interrupted them form doing so. Later in the story, Kosaki and Raku end up confessing to each other, but it resulted in Kosaki getting rejected. Both of them were said to be childhood playmates along with Chitoge Kirisaki.
Chitoge Kirisaki is one of Kosaki's closest friends from childhood and now. They quickly became really good friends, along with Ruri. They would sometimes talk about their love interests, and they would seldom seek advice from each other. When they were children, she and Chitoge played together with Raku. There are many speculations that believe that Chitoge gave Kosaki the picture book.