In the manga, we learn right off that Sunako was rejected by a boy when she was young (middle school age). He calls her ugly, and so since then, she has been a horror loving girl, sinking deeper into the darkness until she gets paired with three extremely handsome guys. In the manga, various issues pop up that cause her to get into situations where she needs a little help.
Its usually Kyohei to the rescue. For some reason, Kyohei has taken on the task of being her friend. Perhaps he sees something in her that no one else does, but he goes to great lengths to protect her. He keeps her from diving off a balcony, saves her from crowds and such regularly, shelters her from a shattering window, and takes on the task of being the lips that kiss her to get rid of ghost possessions and such. Sunako and Kyohei end up in truly odd situations that make them end up having to spend a Christmas alone together, and also live together in a small hut for a bit of time. Sunako usually only sprays blood when Kyohei gets close. This is also true in the drama, but instead of blood, she's pretty much the only person she head-butts.
Unlike in the drama where Kyohei needs to call her ugly to make her mad first, in the manga, Sunako willingly comes to Kyohei's rescue from time to time and uses her violent skills without any real restraint.
In the manga, I think Sunako sees Kyohei as more of a best friend, and her other house mates as good friends in their own way. She doesn't actually appear to think of him as anything more until the much much later chapters.
In the drama though, she follows his commands unquestioningly, wears the bracelet that he gives her, and even recalls his advice when its needed in a situation. Kyohei seems to be the only one who can talk sense into her, and yet also keep her from doing dumb things, not only by force, but also with his words.
It can also be stated though in the drama that the two have similar pasts. Kyohei was ridiculed and made to hide by how handsome he looked. His own parents appear to have named him a bother. This is also why he can't seem to hold a job. He can't get by without his fans gathering. On the other hand, Sunako's character in the drama is much deeper than it was in the manga. It wasn't simply the boy who called her ugly, but since she started to like dark things, her classmates also began to call her ugly. This was probably quite enough to drive her into complete seclussion. This is probably why the two have been able to find some common ground.
Still, her love of darkness doesn't mean she doesn't understand people. She can tell quite well when her housemates need a friend or if they're just being totally stubborn. She isn't afraid to give them a little nudge in the right direction, but she stills remains pretty close off to herself.
As a character, she was probably weak from the start. However, when she met the four boys, her character rebelled, making her weak at times and strong beyond fathoming at others. She understands the ways of the world, but chooses not to take part in them. Though she loves things that others would run from whole heartedly, she can also play the part of someone in the light from time to time (In the manga, it bothers her much less than it does in the drama). Her liking for chocolate also suggests that she still has quite a complete human side, even if it isn't totally "normal."