Malvin and Malcolm are the twin sons of Aunt Gretchen, cousins to Greg, Rodrick and Manny, and the nephews of Susan, Veronica, Cakey and Audra. They are very wild and their mother kept them on child-leashes when they were younger. They are also quite mischievous and while they did not interact much with Greg, it can be presumed they cared little for him, as they scared Greg while he took a shower and then proceeded to take pictures of him naked.
- Their only appearance so far is in Hard Luck, where they play a fairly significant role during Greg's spring break. He mentions how they did dumb stuff in the previous year and caused a lot of stress for the Heffley Family, like when Malvin got sent to the ER for getting hit in the forehead while playing catch with a chunk of concrete. Then Malcolm shaved using Frank's razor and nobody could do anything about it. But in the current events, Malvin and Malcolm only stayed 2 days because Malcolm called 911 after the Heffleys ran out of ketchup, so Frank told them they had to leave. They own lots of pets, including a rabbit that was said to be a male but gave birth to a litter unexpectedly.
- Malvin & Malcolm seem to get into a lot of fights and play rough, frequently hurting each other. However, they seem to get along at times, such as when they played a prank on Greg.
- Because of their behavior and the fact they use child leashes, it is possible that they might have autism or ADHD.
- The first syllable of their names "Mal-" does appear to reflect their nature, as the prefix "Mal-" is a Latin root word meaning "bad".
- Diary Of A Wimpy Kid: Hard Luck (First and only appearance)