Is your angel male or female? Or perhaps neither, who knows, with angels!
You could, maybe, name him/her after someone in your life who is or has been an important, helpful person for you.Friend, relation, perhaps even a writer who has been a good influence.
Or you could use a name from nature..Seashell, Eagle, Hart.
Is the tone of your story serious? Or somewhat lighthearted? Is your angel a warm, helpful presence? Or someone who fills you with awe and holy fear? In the bible, an angel's first words are often, "Fear not!" which makes us think that the person seeing them is, indeed, frightened. But sometimes, angels reveal themselves only gradually.