Mrs. Derkins is the mother of Susie Derkins. She was seen in one strip from the waist down. She also was referenced by Susie in another strip.
Mrs. Derkins also appears off-panel when Susie goes to Calvin's house to ask for eggs and she asks if they had an egg that Susie could borrow, but Susie, meeting Stupendous Man, said no one was home. In yet another strip, she called Calvin's Mom because Calvin dropped a bowling-ball-sized snowball on Susie's head from the top of a tree as Stupendous Man.
Another plot of Calvin's was to simply walk up to the Derkins residence and bash the snowball atop Suzie's head. However, when Mrs. Derkins answered the door, Calvin changed tack and furtively claimed to be a snowball salesman. Calvin walked away remarking to himself that his "Plan A's" are superb, but that his "Plan B"s leave a lot to be desired.