Help to understand this conversation

Question

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.

1. Who said "Good evening,"? Is it Nick?

2. 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?

Answer 1

The author has just finished describing Nearly Headless Nick, and then writes "'Good evening!' he said, beaming at them." From this, you can gather that the "he" here is referring to Nearly Headless Nick. If the writer intended for this to be someone different, than it would be badly written, since the context lends itself to the last person being referred.

Harry, apparently having a horrible evening (emptying his shoe of water in emphasis to this fact), responds, "Says who?" In other words, who's claiming it's a good evening? Certainly not him. It's a colorful way of saying that for Harry, it's quite the opposite of a good evening.

The text in general seems to be creating the scene of the dining hall prior to dinner. The meeting with Nearly Headless Nick was probably used to establish that Harry is not of high spirits, but he was at least happy to be present for the sorting hat ceremony which is about to begin.

I hope that clarifies it. If not, ask in the comments and I'll adjust my answer.