This unnamed angel was a member of Castiel's garrison.
For 2,000 years, this angel watched over mankind from a distance, as was the order given to her garrison. After her captain, Anna Milton, abandoned Heaven in order to join mankind, Castiel took charge, becoming this angel's new leader.
She most likely took part in the mission to raid Hell in order to retrieve Dean Winchester's soul. Later on, she was approached by Uriel, who offered her a chance to rebel against Heaven and serve Lucifer instead, but she refused, and was subsequently killed.
This angel was not the first member of her garrison to be given such a offer and die when she refused it. At least six others were killed. Upon finding her corpse, Castiel was determined to locate the culprit, whom he believed to be among the demons, and decided to find the answer by torturing Alastair, and then passing on the task to Dean, when Alastair proved resilient.
Once Alastair confessed that the demons were not responsible for the murders of Castiel's subordinates, Uriel himself decided to reveal the truth in order to beckon Castiel to join his side as well. Castiel's refusal led to a severe beating, and Castiel nearly died had his former superior, Anna, not arrived in time to stab Uriel through the back of the neck, the same way he had murdered their siblings.
The death of Uriel meant this angel - and six of her companions - were avenged by the very angel whom they once served, Anna.
This angel's vessel was a blonde haired woman donning a white dress, which is unusual for an angel given that most angels are shown dressed in either casual wear or formal suits. Upon her death, her large wings were burned into the pavement beneath her, as revealed by a helicopter flying over her.