Virginia Otis is only called an angel once during the story. The line is spoken by the Duke and is the last line of part 6.
"What an angel you are!" cried the young Duke, and he put his arm round her neck, and kissed her.
A little bit before the Duke speaks this line, the entire family had been frantically searching for Virginia. They couldn't find her anywhere. The reason for that is because she was helping Sir Simon pass on to his eternal resting place. She is the "golden girl" that needs to pray for Sir Simon's soul. She is good, sweet, and gentle, and that is why the angel of death has mercy on Sir Simon. Those definitely sound like angelic qualities; however, the Duke also believes that Virginia Otis looks like an angel after she shows the family the skeleton and explains that God has forgiven Sir Simon. She rises to her feet at this moment, and a light of some sort illuminates her face. She is like a glowing angel of God.
"God has forgiven him," said Virginia, gravely, as she rose to her feet, and a beautiful light seemed to illumine her face.