Since most readers of romance novels are women, the heroine is the key to a great romance book. She needs to become the alter ego of the readers.
The heroine needs to me attractive, strong and smart, but at the same time, she needs to be vulnerable and emotional. She needs to be the kind of person that the readers of the novel would enjoy spending time with.
A hero in a romance is usually exactly that, a hero. However, this doesn’t mean that he has no flaws. If he is not vulnerable, there will be no place for the heroine in his heart. A hero needs to be someone that, despite all his flaws and issues, the heroine and the readers can fall in love with.
At the same time, both the heroine and the hero need to be human beings with complete and complicated inner lives. There has more to be going on than just sexual attraction. Every character in a romance novel has an independent voice.
This being said, it is the emotional tension that drives every romance story. The story usually comes from who the heroine and the hero are, what keeps them apart and what needs to happen for them to be together. It is possible that the hero is wealthy and he has a hard time accepting that the heroine is not after his money. It is also possible that the hero has some emotional issues that prevent him from getting into a relationship with the heroine, such as his mother leaving his father or all his relationships lasting for a very short period of time. In all romance books, the tension starts with personalities or the past and then grows as the hero and the heroine go through various situations, act and react to them.
In every romance novel, this emotional tension is unique, which is why the genre of romance novels is so popular. The more emotional tension a romance novel has, the faster the readers read the novel. It is also very important for romance novels to have the kind of tension that readers can relate to. The more deeply a reader feels, the more often she will be thinking about the book and reading the book. Such novels are called page-turners for a reason.