Dumbledore is Hogwarts' former headmaster. He has recently died. Dumbledore was the one man, who Harry looked up to, and he is considered by some to be one of the greatest wizarding minds of all time. Harry has a closer relationship with Dumbledore than most other students.
Dumbledore has laid careful plans to help Harry in the search for Horocruxes. He is a very secretive and untrusting man, but wants Voldemort defeated.
After Dumbledore's death, his ethics and stands are called into question. Dumbledore was briefly misguided as a teenager, and his sister paid the price for it. He has been tormented by her death ever since.
Harry Potter and the Deathly Hallows