Kyoko is a very good student and is competing for the SOY (Student Of the Year) in her school. Her competition is Tamaki Renjou.
When Kyoko's mother injured her ankle, Kyoko had to go to the Renjou family house in her place and clean and keep it (and its inhabitants! ;) That's how she met Tamaki, Mito and Shizuka.
She grew alone with her mother, who seems not to give her much attention, and never knew her father. As the plot advances, she will learn more things about it thanks to the Renjou family.

Her favorite food is fruit, and as a note, she uses E cup bras ^^;;; (seriously! I'm not kidding, it says it in the manga. And me thinking it was just that the mangaka liked to draw them big... ^^;;; )

Mito is a very friendly and famous guy in school. He is the star of the soccer team and an ace in it. Of course all the girls love him for that.
At the start of the story he has a girlfriend, but she is a nice girl and will let him go when she sees that he likes another much more than what he likes her.

From the three brothers, he is the most easy-going and has no secrets like the others.

Tamaki is a serious guy and a very good student (though he also has his moments! ;). Kyoko at first wanted to hate him very much because he is her competitor for the SOY title, but then she fell in love (or so the thought) with him.

Real spoilers here, if you want to see them, select the next white spaces with your mouse button! (Thanks to Rika for this trick! ^^ )
Tamaki had an affair with Yuma, who is the fourth wife of his father (though not related to any of the brothers). Even if that relationship ended, he is still in love with her and cares very much for her.

Shizuka is the younger of the three brothers and can be pretty petty when he wants to. But he has a reason for it, and when Kyoko helps him, he falls in love with her.
He likes playing video games a lot, and later on in the story he will start falling for a girl who is supposed to be his fiancee.

Real spoilers here, if you want to see them, select the next white spaces with your mouse button! (Thanks to Rika for this trick! ^^ )
Shizuka was sexually harrassed by his female teacher (but not raped, he collaborated very willingly), and when she re-appears Kyoko helps him in sending her away so she doesn't disturb him any more.

Yuma-san is the fourth wife of Tamaki, Mito and Shizuka's father. She is 28 years old and is really very pretty. She is not related by blood to any of the brothers, of course :)

When we first meet her in the manga, seems that she and the brothers have a strained relationship, especially with Tamaki... Soon Yukino will discover what is happening around in that strange house of the Renjou family...