I believe it transforms into Jacob Marley as a sign of whats to come.
In "A Christmas Carol" by Charles Dickens, the knocker plays a significant role in the story's supernatural elements. The knocker takes on a ghostly form, first transforming into the face of Jacob Marley and then into a gruesome specter. This reflects the themes of haunting and redemption present throughout the novella.
a ghostly vision of Jacob Marley
His nephew
fezziwig
Fred
Ebenezer Scrooges
Scrooges house keeper
Bah, Humbug!
He was Scrooges clerk
Her name was Fan
He was a trader and money lender
he is scrooges nephew. he loves Christmas but his uncle hates it.