A topic sentence in each paragraph of an essay ensures that the person writing the essay stays focused on the relevant information that he wants to include in that paragraph. The topic sentence always links back to the thesis statement in some way and is, in fact, the most important sentence in a paragraph. It also prompts the writer to find other information to add to that paragraph about the relevant topic. For example, say a writer chooses to write about Lord of The Flies and, as this question is about a character, is writing a paragraph about Ralph. A topic sentence could be:
Ralph is the boys' natural choice for leader on this island without "grown ups."
The thesis may have said something about survival or choices and so this topic sentence links back to that. The writer can then add some description of Ralph and follow this with the reasons why Ralph is the best choice and how he came to be nominated in the first place. Details of whom he is competing against are also relevant and whether the choice of Ralph will drive the plot of the story, improve the story, change the pace of the story, change Ralph's life forever and so on, all of which contribute to a good paragraph with an appropriate topic sentence. This topic sentence therefore controls what information will be in the paragraph and also links to other important elements of Lord of The Flies because the conch is representative and symbolic of democracy and good order (and this links back to the thesis).
As another example, a writer writing about The New Adventures of Sherlock Holmes could have a topic sentence like:
Sherlock Holmes is a character who is intriguing and strange in his methods of crime-fighting and problem-solving.
The writer could then describe his looks, why people are drawn to him, his aptitude or genius in gathering information, his conflicted life and involvement with the sordid side of London, drugs and whether his involvement drives the plot, changes the pace, improves people's lives, and so on. This could be linked to a thesis statement about different ways of solving crimes, for example.
Topic sentences are fairly general in scope (in terms of the specific topic) so that not only do readers keep reading, but also they can follow a logical flow, making the information easier to understand and remember. When choosing your topic sentence about a character, make a list of the characteristics that you have become aware of while reading about this character. Perhaps the character has been described by various other characters or the narrator has drawn attention to something about that character. Think of adjectives to describe the character such as kind, compassionate, inspiring, selfish, mean, cruel, clever, believable, special, unusual and also adjectives and nouns that give physical information such as blue eyes, long legs, brown hair, scar, a limp and so on. Then, consider words that link your character to the story such as protagonist (main character), narrator (the one telling the story), victim, victor, involved, distant, etc. From all these words, you can then put together a topic sentence which will lead you into all these descriptive elements.
You have to write about the character in a topic sentence you introduce the character by telling the back-round of the character. You have to introduce the character by telling how when and what the character is about.