The angel donut is a white donut with white frosting, golden sprinkles, angel wings, and a halo.
Role in episode
The angel donut appears after Patrick steals SpongeBob's donut, and interrogates Patrick for his actions. The angel donut and the devil donut seem to act like Patrick's conscience. The angel donut tries to stop Patrick from hiding the donut from SpongeBob, while the devil donut tells Patrick to not listen to the angel donut because it is covered with sprinkles.
The angel donut admits that it was a good idea after the devil donut explains the plan, which is to attach the donut to a chain so Patrick can pull it out when SpongeBob arrives. This plan, however, fails. When Patrick pulls the chain out of his mouth, the donut is no longer attached to the chain.
This is reminiscent of the popular media nicknamed "Psychomachia," where a character's conscience (i.e: Good, telling good advice and Evil, telling bad advice) tells the character what to do in a certain situation.