Who is Simonov from Notes from the Underground and what is their importance?

Simonov is a former schoolmate of the writer and the one person he feels comfortable occasionally visiting, though he notes that there are times when he's not at all certain he's welcome. He's among the three throwing the farewell dinner for Zverkov. Before the end of that dinner, he is also angry at the writer. The writer later sends a note of apology for his actions during that dinner, seemingly in an effort to keep this one person who seems to be the one person the writer claims as a friend.