Normally, a biographical essay is one in which we write about the life and personality of a person who actually lived. A biographical essay should normally contain a real-life person, a thesis to state the specific aspect of the person's life you will be focusing on or analyzing, details about major events in the person's life, and a portrayal of the person's character traits. The interesting thing about Thomas More's Utopia is that it actually does contain characters named after real, historic people, such as Sir Thomas More himself, Peter Giles who was a humanist philosopher, and John Morton who was one of England's chancellor's. However, while More included biographical information in the book about each of these characters that is the same as the biographical information as the real, historical figures, it's important to keep in mind that More did not intend to represent the views of the actual historical figures in his book, including his own views; instead, More created characters using these historical names, and the characters should be thought of as all other fictional characters. Therefore, depending on exactly what you need to do for your assignment, you will choose one of these historical names and either use Utopia to piece together the biographical information of the character disclosed in the book, or you will research the historical figure's true biography and write about that instead. But keep in mind those two options will not create the same essay, so it's very important to know your exact instructions for this essay. You will either need to write about the real, historical biography of one of the historical figures named in Utopia, or you will write about the fictional biography of one of the figures disclosed in the book.
For example, we learn in the very first chapter that the narrator, Sir Thomas More, was sent by King Henry VIII to Flanders as an ambassador to sort out a serious disagreement Henry VIII was having with Charles, the Prince of Castile, as we see in the opening sentence:
Henry VIII, the unconquered King of England, a prince adorned with all the virtues that become a great monarch, having some differences of no small consequence with Charles the most serene Prince of Castile, sent me into Flanders, as his ambassador, for treating and composing matters between them.
We know that all of these details are indeed historically accurate for the real Sir Thomas More's biography. Sir Thomas really did go to work for King Henry VIII who really did send Sir Thomas to the Netherlands as an ambassador, and Flanders is in Belgium, which is even still today a part of the Kingdom of the Netherlands. Hence, if you chose to use Utopia to write your biographical essay on Sir Thomas More the character, these are certainly some details you would include, which happen to coincide with the biographical details of the historic Sir Thomas More.
Hence, to recap, for your biographical essay, you could choose to write on one of three characters: Sir Thomas More, Peter Giles, or John Morton. Plus, depending on the directions for your assignment, you will either be exclusively using Utopia to piece together the biographical details More gives for each of these three fictional characters, or you will be researching the real biographical details for one of the real, historical figures.