nephilins:

“[…] Like the fact that the person Sirius cared most about in the world was you,” said Dumbledore quietly. “Like the fact that you were coming to regard Sirius as a mixture of father and brother. Voldemort knew already, of course, that Sirius was in the Order, that you knew where he was — but Kreacher’s information made him realize that the one person whom you would go to any lengths to rescue was Sirius Black.”