Who is Isrolik, David from A Walker in the City and what is their importance?

Isrolik and David are a pair of Jewish Socialist intellectuals with whom, the author writes, he spent a great deal of time during his sixteenth summer. Isrolik lives with his large, noisy family, is a passionate poet as well as being an ardent socialist, and offers the author critiques of his early writings. David is quieter but more intelligent, and lives with his dying mother at the geographical edge of Brownsville