Arianna, his sister.
A fight broke out after an argument between Grindelwald and Dumbledore during their youth. Grindelwald and Dumbledore had a duel and Dumbledore believes he might have killed his sister by accident.
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There is no evidence Dumbledore killed Ariana. It could have been any of the three of them; Albus, Aberforth or Grindelwald. Dumbledore tells Harry that his greatest fear was that he struck the blow that ended her life, but Harry chooses not to ask whether he ever found out who was responsible for killing her, so we don't know. In fact, there is nothing any where that says Dumbledore killed anyone and considering his disdain for murder and the value he placed on intact souls (which are ripped apart when one kills), it is not likely he ever did.
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Yes-, incorrect no, he defeated Grindlewald then the ministry of magic locked him in his own prison for "mudbloods" and muggles, called Nurmengard. Voldemort kills Grindelwald near the end of the seventh book.
No, Dumbledore died in Harry Potter and the Half-Blood Prince. Umbridge was still alive in Harry Potter and the Deathly Hallows. She would have died in Azkaban as she was imprisoned for life for her crimes against muggle-borns.
according to J.K. Rowling, yes. She said in an interview that Dumbledore is a homosexual, but once he defeated Grindleward and sent him to prison, Dumbledore led a celebite life.
Dumbledore's full name is Albus Percival Wulfric Brian Dumbledore, giving him three middle names.
Albus Percival Wulfric Brian Dumbledore and his brother was called Aberforth Dumbledore He also had a sister called Ariana Dumbledore.
Albus Dumbledore was a half-blood.
Albus Dumbledore is male.
Aberforth Dumbledore is his full name. No middle name has ever been mentioned.