There is actually a lot of development in this chapter, so much so that I didn't know what to include in my previous post. That, and I was in a hurry.
The backstory is revealed. Marika always stayed at home alone because of her weak body. And then, one day, she meets a little boy; Raku climbed the tree outside of her house. (They are so cute! Raku is so innocent.)
Later, when they are playing together in her room, Marika asks Raku what kind of girl he likes. As a little boy, he doesn't know what the meaning behind the question is. He randomly blurts out someone with long-hair.
Back to the present time, Chitoge, Kosaki, Raku, and Marika discuss about the promise. Marika confirms that they all (Chitoge, Kosaki, Raku) do not remember anything about the promise. They also show the keys to each other.
She says to them, "well, naturally, the promise is about marriage." 8D
She also brings up the fact that she and Raku had already kissed. 8D
There is too much to summarize in this chapter.
Here is Kosaki's reaction after she hears about their kiss.