I want to echo #2 in focusing on the work of Kate Chopin as a great example of both "what" and "who" when we think of the transition period between Romanticism and Realism. Certainly she was a forerunner of realism in Romantic times whose work was never recognised much in her life time. In particular, her major magnus opus, The Awakening, was scorned and rejected by critics because of its focus on issues that were just too real, such as adultery and dissatisfaction in marriage.
Well, in terms of "what," local color writing comes to mind because it served as a literary bridge between romanticism and realism in American literature, appearing just after the Civil War. Local color writers wrote about their own particular parts of the country, capturing their own locale's unique elements. The themes of local color stories were frequently romantic, but the attention to very realistic details in capturing settings, characters, cultures, and daily life paved the way for the realists to come. For example, vernacular style, writing dialog to capture the sound of how people actually spoke, developed in local color writing.
Local color writing developed after the Civil War because people had become more aware of different parts of the United States. Transportation had improved and people were moving westward; also, during the Civil War, many people had been uprooted from their homes to live in other areas, and soldiers from both the North and the South had traveled far from home, seeing parts of the country they had never experienced. Local color writing appealed to Americans who wanted to know what life was like in places they had never seen. This desire led local color writers to write very realistically in detailing people and places, while still developing romantic themes. Eventually, modern writers dropped the romantic elements in fiction and presented all of life realistically as they saw it.
The American Realism era occurred between the Romantic and Modern periods; so essentially any American Realist author would serve for an answer to your question. However, if you are looking for authors who represent elements of Romanticism and Modernism, writers such as Stephen Crane, Ambrose Bierce, and Kate Chopin are transitionalists. Crane couples the idealism of Romanticism (found in several of his characters at the beginning of his works) with the cynicism and irony of Modernism. While Crane is certainly a Realist by definition, his Red Badge of Courage helped pave the way for Modern American novels.
Bierce, the ultimate cynic, uses idyllic nature much in the same way that Romantic authors do but then combines that element with the disillusionment found in many Modern author's works (Steinbeck, Fitzgerald, etc.). "An Occurrence at Owl Creek Bridge" is an example of such writing.
Similarly, Kate Chopin uses a Realist's style to discuss controversial modern issues (women disillusioned with marriage, multi-racial marriages, and divorce--to name a few). Thus, while many of her stories are set during the Romantic time period, their themes are best associated with the 1900s.
To come up with other authors, consider American writers from around 1850-1900.
oh sorry. your answer is good, but i made a mistake it should be what not who in my question.