Essay option #4 on the Common Application states, "Describe a character in fiction, a historical figure, or a creative work (as in art, music, science, etc.) that has had an influence on you, and explain that influence." As with each of the essay options, this one comes with its own opportunities and pitfalls. Weak essays tend to spend too much time working to "describe" the character and too little time on the more important part of the prompt ("explain that influence"). Colleges require an essay so that they can learn about you, so your essay needs to use the fictional or historical character to illuminate your own personality and passions. They don't want a school report on Hamlet or Abraham Lincoln, but an exploration of how those figures changed, motivated or inspired you.
This question gives you lots of room to be creative, so it's often wise to steer away from predictable choices like Albert Einstein or George Washington. At the same time, you don't want to trivialize the task by focusing on a ridiculous character. Writing about South Park's Mr. Hankey the Christmas Poo might be funny, but you need to ask yourself if you're really giving the admissions officers a positive portrait of yourself.
I've see lots of advice to avoid writing about cartoon characters, but they can work if handled carefully. Read Felicity's essay "Porkopolis" for an example -- she writes a surprisingly successful essay about Lisa Simpson. For a character in a novel, read "Wallflower," Eileen's essay.
For more information, read these seven tips for essay option #4 and this overview of the six essay options on the Common Application. For tips on all the Common Application options, check out these articles: