Who is Thomas Schell, Jr. from Extremely Loud and Incredibly Close and what is their importance?

One of the novel's secondary narrators, Thomas is Oskar Schell's missing paternal grandfather, a native of Dresden, Germany, who speaks German, Greek, Latin, and English. In his youth, he believes himself already to be an accomplished sculptor. He loses everything in the Allied firebombing of Dresden at the close of World War II, including his parents and pregnant girlfriend, Anna Schmidt. Seven years later, having suffered extreme aphasia (loss of language), he meets Anna's sister in a New York bakery and accepts her proposal of marriage. To make life manageable, Thomas tattoos his hands YES and NO and constantly carries a daybook in which to write questions and responses. Thomas and his neurotic wife live a life of strict rules, haunted by his love of Anna.