What does it refer to or what role does it take?

Question

   One day in July, Aunt Petunia took Dudley to London to buy his Smeltings uniform, leaving Harry at Mrs. Figg's. Mrs. Figg wasn 't as bad as usual. It turned out she'd broken her leg tripping over one of her cats, and she didn't seem quite as fond of them as before. She let Harry watch television and gave him a bit of chocolate cake that tasted as though she'd had it for several years.
   That evening, Dudley paraded around the living room for the family in his brand-new uniform. Smeltings' boys wore maroon tailcoats, orange knickerbockers, and flat straw hats called boaters. They also carried knobbly sticks, used for hitting each other while the teachers weren't looking. This was supposed to be good training for later life.
   As he looked at Dudley in his new knickerbockers, Uncle Vernon said gruffly that it was the proudest moment of his life. Aunt Petunia burst into tears and said she couldn't believe it was her Ickle Dudleykins, he looked so handsome and grown-up. Harry didn't trust himself to speak. He thought two of his ribs might already have cracked from trying not to laugh.
(Harry Potter and the Sorcerer's Stone)

What does it refer to or what role does it take?

Answer 1

"It was her Ickle Dudleykins" is similar to "It's me." you could get an answer when you ask "Who is at the door?" It's a dummy pronoun used because a sentence in English always needs a subject.

Imagine a person I care of comes to my house, after I have not seen her for years. I could say "I cannot believe it's you!" which could be rephrased as "I cannot believe you are standing at my door!"
In the same way, the quoted phrase could be rephrased as "she couldn't believe the person she was looking at was her Ickle Dudleykins." In that way, there would not be any need to use a dummy pronoun, but it would change the phrase in something native speakers would not probably say.