The Great Hall looked its usual splendid self, decorated for the start-of-term feast. Golden plates and goblets gleamed by the light of hundreds and hundreds of candles, floating over the tables in midair. The four long House tables were packed with chattering students; at the top of the Hall, the staff sat along one side of a fifth table, facing their pupils. It was much warmer in here. Harry, Ron, and Hermione walked past the Slytherins, the Ravenclaws, and the Hufflepuffs, and sat down with the rest of the Gryffindors at the far side of the Hall, next to Nearly Headless Nick, the Gryffindor ghost. Pearly white and semitransparent, Nick was dressed tonight in his usual doublet, but with a particularly large ruff, which served the dual purpose of looking extra-festive, and insuring that his head didn't wobble too much on his partially severed neck.
“Good evening,” he said, beaming at them.
“Says who?” said Harry, taking off his sneakers and emptying them of water. “Hope they hurry up with the Sorting. I'm starving.”
The Sorting of the new students into Houses took place at the start of every school year, but by an unlucky combination of circumstances, Harry hadn't been present at one since his own. He was quite looking forward to it. Just then, a highly excited, breathless voice called down the table.
There are a couple of things I don't understand from the conversation above.
Who said "Good evening,"? Is it Nick?
Why Harry said "Says who?"
I don't really understand what the purpose of this conversation is. To me, it's not that relevant to the whole context here. Can anyone help me with it?