Why he use "is" instead of "am"
“Dobby has known it for months, sir. Harry Potter must not put himself in peril. He is too important, sir!” Dobby said to Harry.

Why did't he say " I have known it for months, sir. You must not put yourself in peril. You are too important,sir." instead?

Question

Why he use "is" instead of "am"
“Dobby has known it for months, sir. Harry Potter must not put himself in peril. He is too important, sir!” Dobby said to Harry.

Why did't he say " I have known it for months, sir. You must not put yourself in peril. You are too important,sir." instead?

Answer

Anyhow, Dobby is a slave so that is why he uses his first name instead of the pronoun "I" and why he uses Harry's name instead of the pronoun "you".

Answer

The way you wrote it is how a person would and should say it, but characters who are slaves or who are insane often times speak of themselves in the third person singular. Sometimes they even speak in the 1st person plural if they are insane and have multiple personalities. Take for instance Gollum from Lord of the Rings. He e en says stuff like "It gots the precious. We must kills it." "It gots the precious" should be "Frodo has the ring." and "We must kills it" should be "I must/have to kill him." Gollum is insane and speaks to himself though using "we" instead of "I".

Answer

Dobby talks funny. It's not for learning English.