He worries that he is the weapon Voldemort is developing.
Mrs. Weasley notices that Harry looks pale and sends him to nap before dinner.
But he doesn't sleep – he paces around worrying, "this is insane – if Voldemort's possessing me, I'm giving him a clear view into the Headquarters of the Order of the Phoenix right now! He'll know who's in the Order and where Sirius is ... and I've heard loads of stuff I shouldn't have, everything Sirius told me the first night I was here" (23.17).
Harry realizes that the only way he can keep everybody safe is to run away.
Harry is nearly out the front door with his trunk when he hears a voice.
It's Phineas Nigellus speaking from his portrait.
He mocks Harry for being a coward: "I thought [...] that to belong in Gryffindor you were supposed to be brave?" (23.18).
Phineas Nigellus then passes on a message from Dumbledore: "Stay where you are" (23.29).
Harry doesn't think much of this advice but he sticks around Number Twelve, Grimmauld Place.
Harry is afraid to sleep, and he's afraid to be around his friends. He stays in hiding in Buckbeak's room for much of the next day.
He is surprised when there is a loud knock on the door.
She was supposed to be skiing with her parents, but she has changed her mind and decided that she will spend the holidays at Number Twelve, Grimmauld Place, with Harry and the Weasleys.
She knows what Harry overheard the night before, and she thinks he's being silly.
Hermione leads Harry to Ron and Ginny.
Ginny flares up angrily: "Well, [hiding] was a bit stupid of you [...] seeing as you don't know anyone but me who's been possessed by You-Know-Who, and I can tell you how it feels" (23.66).
(Ginny is referring to Book Two, when she was possessed by Voldemort through his school diary.)
Harry apologizes and asks how he would know if he is possessed.
Ginny asks, "Well, can you remember everything you've been doing? [...] Are there big blank periods where you don't know what you've been up to?" (23.71).
Harry can't think of anything, and he starts to relax.
He realizes with relief, "I'm not the weapon after all" (23.82).
On Christmas morning, Harry gets loads of presents.
Fred and George warn them not to go downstairs, since "Mum's crying again" (23.90).
Apparently, Percy returned the sweater that she knitted for him for Christmas without even a note.
So, they decide to stay upstairs until Lupin finishes reassuring her.
Hermione has got a present for Kreacher that she intends to bring to his den under the boiler.
They go and explore Kreacher's living space.
It's lined with blankets and filled with stale crusts and relics from the Black family.
There is a photograph of Bellatrix Lestrange that Kreacher has put in front of all the others.
Hermione decides to leave the present for him to find later.
Sirius suddenly realizes, "Come to think of it [...] has anyone actually seen Kreacher today?" (23.111).
No one has seen him for ages.
Harry worries that, when Sirius said "out" (23.115), Kreacher took that as permission to leave the house.
Harry points out, "[House-elves] can leave the house of they really want to [...] Dobby did, he left the Malfoys' to give me warnings three years ago. He had to punish himself afterwards, but he still managed it" (23.117).
Sirius looks disturbed at this news, but then dismisses it.
After lunch, they all troop off to visit Mr. Weasley at St. Mungo's.
Mr. Weasley has been experimenting with Muggle medicine – stitches — and Mrs. Weasley flips out.
To escape her wrath, Ron, Harry, Hermione, and Ginny all go racing off to find a cup of tea.
As they are walking through the hospital, they see a familiar face in the SPELL DAMAGE wing.
It's Gilderoy Lockhart, their former Defense Against the Dark Arts instructor.
Lockhart is still obviously suffering from the memory charm he cast at the end of Book Two.
Harry and the rest get roped into visiting Lockhart for a bit.
Lockhart is certainly very cheerful and friendly, though he's still as vain as ever.
But as they visit Lockhart, they see someone completely unexpected in this wing of the hospital: Neville Longbottom, accompanied by an elderly witch.
Harry realizes exactly why Neville is there and tries to distract his friends, but it's too late: Ron jumps up and yells, "Neville" (23.187).
Neville's grandmother (that's the elderly witch) greets all of them.
She is surprised that Neville hasn't told them about his parents.
She announces, "Well, it's nothing to be ashamed of! [...] You should be proud, Neville, proud! They didn't give their health and their sanity so their only son would be ashamed of them, you know!" (23.200).
It turns out that Neville is at St. Mungo's visiting his parents, Frank and Alice Longbottom.
They were both tortured so long with the Cruciatus Curse at the end of the last war with Voldemort that they went insane.
So now, they live at St. Mungo's in the ward for spell damage.
After Alice Longbottom hands her son a gum wrapper, "Neville looked around at the others, his expression defiant, as though daring them to laugh, but Harry did not think he'd every found anything less funny in his life" (23.210).
Ron, Hermione, and Ginny are shocked, but Harry admits that Dumbledore told him in Book Four: "I promised I wouldn't tell anyone ... that's what Bellatrix Lestrange got sent to Azkaban for, using the Cruciatus Curse on Neville's parents until they lost their minds" (23.218).