Limiting the number of characters is a tool to gain intensity. Most romance novels are around 200 pages or less. With so little space there must be a concentration on the heroine, one or two close male characters and no more than one antagonist who is out to spoil things for the couple. Anymore than that would be a distraction.
Creating a fictional character requires detail and consistency. Write a short history for her, both private and professionally, take items from her life story and write a short scene describing how she looked, what she said, what she was thinking and feeling. Take the things you have used in these inventions into scenes in the book you are writing. The reader can sense where you have done this background work and where you are winging the details on the spot.
Every heroine requires an antagonist, someone or something that is acting against her interests and wishes. This can be another person, or it could be an internal conflict. If it is the later, be sure to have it represented in the real world -- the conflict needs to be seen and felt as real by the reader.
Change is an important tool in romance novels as it provides meaning to the story. The character must change in the course of the book as a result of the conflict they go through. There is no point if there is no development in her character. She should be different in an important way as a result of what has happened to her.
The reader identifies with the character only if she can find the setting credible, that means she will not suspend her disbelief unless the setting is realistic -- so write about what you are sure about and do the research if it is an historical novel. Use objects in a person's room to give the reader a sense of their character -- how people live says a lot about them and readers are quick to pick up on these clues.
Writing the novel in the first person allows the reader to experience the novel as the heroine is experiencing the events herself. Be sure to provide the physical descriptions and direct feelings that are necessary for this. Give dialogue and show action and reaction. Restrict narrative summary to quick links between the main scenes of the novel.