Онодера Косакі

Both Kosaki and Raku are completely unaware that the feelings they hold for each other are mutual. Like in many average fictional love stories, these two try not to show their feelings to each other. She loves Raku because he is kind and caring. Just like the other girls who like him, she can be jealous as well but not to the point of having bloodlust. She often worries/wonders whether Raku and Chitoge's "fake relationship" is actually developing into something more than just that.

Chitoge and Kosaki have a really trusting relationship. It can even be considered that they are best friends since Chitoge only tells her problems to her (except in some cases, Raku as well). Despite her knowing that Chitoge and Raku are only pretending to date, she at times can't help but be suspicious of their relationship.

In the end of the manga, Kosaki is revealed to be The Promise Girl, who had made a promise of marriage with Raku when they were young.

(Source: Nisekoi Wikia)