Oskar Schell is the nine-year-old protagonist of Extremely Loud & Incredibly Close. He is an eccentric, intelligent, and clever young boy who self-identifies as a number of things including inventor, amateur entomologist, origamist, and amateur archaeologist. He often contemplates deeper topics and shows great empathy beyond what the average 9-year-old might have. His thoughts have a tendency to trail off into far-flung ideas, such as ambulances that alert passerby to the severity of their passengers' conditions and plantlike skyscrapers, and he has several assorted hobbies and collections. He is very trusting of strangers and makes friends easily, though he does not have many friends his own age. In the film it is alluded that he has Asperger's syndrome. Oskar mentions being taken in for testing in his first interaction with Abby Black, however he states that "...Tests weren't definitive."
Oskar's mother, Linda Schell, referred to as "Mom" by Oskar in the book, cares for her family greatly. After Thomas's death, Linda tells Oskar "I won't fall in love again." Though it is implied that she knows Oskar is running around the city meeting strangers, she nevertheless allows him to do so in order to discover more about his father.
Oskar's grandmother is a kind woman who is very protective of Oskar. She calls out to him often, and Oskar always responds with "I'm okay" out of habit. When she arrived in the United States, she read as many magazines as she could to integrate herself into the culture and language. As Anna's (Oskar's grandfather's first love) younger sister, she enters into a tumultuous marriage with Oskar's grandfather, and the couple breaks up before the events of the novel.
Mr A. Black is an elderly man who is one hundred and three years of age, who lives in the same apartment building as Oskar, and joins him for some of his journey. Prior to meeting Oskar, Mr. Black had not left his apartment in twenty-four years, after having had a rather adventurous life. He is nearly deaf, and cries after Oskar turns on his hearing aids after a "long time" where he was unable to hear.
Oskar's grandfather, Thomas Schell Sr. (also referred to as "the renter") is an important character in the story, even though he does not physically meet Oskar until the book's end. After the death of his first love, Anna, Oskar's grandfather loses his voice completely and consequently tattoos the words "yes" and "no" on his hands. He carries around a "daybook" where he writes phrases he cannot speak aloud. He marries Anna's younger sister, Oskar's grandmother.
Anna is an absent character. She is Oskar's grandfather's first love. Oskar's grandfather falls in love with her instantly. She dies in the Dresden firebombings of World War II after telling Oskar's grandfather of her pregnancy. She is Oskar's grandmother's sister.
Abby Black is William Black's ex-wife. She is forty-eight years old and lives by herself. She is friendly and welcoming to Oskar when he arrives at her house, though she does decline Oskar's offer of a kiss.
Oskar's father, Thomas Schell, dies before the events of the book begin, having been in 1 World Trade Center the day of the attacks. Oskar remembers him as caring, smelling of aftershave and always humming the song "I Am the Walrus" by The Beatles. Thomas Schell organizes several expeditions for Oskar, such as a game to find an object from every decade of the past century. These adventures with his father are one of the reasons Oskar begins his journey about the key.
Stan is the doorman in the building Oskar lives in. He alerts Oskar when he has mail.
Buckminster is Oskar's cat.