Mr. Miggins is Candace Flynn's stuffed teddy bear. Based on his appearance, he has been with her a very long time: Mr. Miggins has been patched at least twice, and has a loose eye and stuffing showing from his seams.
Mr. Miggins' current role is to be a "security blanket" for Candace and resided temporarily in the family's panic room. When she believes that her brothers have become robots, she flees to the panic room and clutches Mr. Miggins while calling her mom. When her mom doesn't come immediately home, Candace talks to Mr. Miggins about the robots and devises a plan for capturing one as evidence to show her mom.
At the end of the day when a robot version of herself is created, Candace runs back to the panic room. The "Candroid" has reached the panic room first and is holding Mr. Miggins. Candace quickly reclaims her beloved bear from the robot. ("I, Brobot")
Later that summer, Candace brought Mr. Miggins back to her room. She was in the middle of trying to figure out what to wear to Stacy's costume party when she decided to ask "Jeremy" (Mr. Miggins with a picture of Jeremy taped to his face) what he thought. His stint as a substitute Jeremy was short-lived when the real Jeremy called. ("Boyfriend From 27,000 B.C.")
Candace talks to Mr. Miggins as if she were talking to herself. Mr. Miggins is very worn down and old, as indicated by the features on the bear.