Arannir was a Wood Elf vampire hunter of the Dawnguard.
Arannir grew up in Shor's Stone, a town in The Rift. He had a good childhood and several friends growing up. He was the son of the local Priest of Arkay and as such knows all of the proper burial techniques. As a young adult, he ended up sleeping with a woman who he had come to know and love as a follower of Dibella.
As it turns out, she was a vampire and while they were "having relations", she bit him and gave him the Sanguinare Vampiris. He thought she was just in the moment until he was tending his garden like he had a thousand times before when he noticed how hot the sun was.
He thought this was odd, as he had lived in the sun all his life but it was never this hot. He soon became quite hungry, and went inside to eat. Soon, his girlfriend came over and began asking strange questions. Arannir felt that something strange was going on; he became angered and asked her to show him her eyes.
He had previously thought they were orange becase she was a Dark Elf, but they were that color because she was a vampire. In a fit of rage, he killed her and prayed that she would go to Coldharbour, where she belonged. After a long time of traveling he discovered that it could be cured. After he was cured, he joined the Dawnguard. He has been with them ever since.