Malcolm is a Muggle boy and a member of Dudley Dursley's gang of bullies who lived in the vicinity of Little Whinging, Surrey. His primary role in the story is to help establish the miserable and abusive childhood Harry Potter endured at the hands of his relatives and their associates before he discovered his magical heritage and departed for Hogwarts School of Witchcraft and Wizardry.
Malcolm is introduced as one of Dudley Dursley's friends who formed a gang that terrorised both Harry Potter and other children in their neighbourhood. Alongside Piers Polkiss, Dennis, and Gordon, Malcolm frequently participated in Dudley's favourite “sport,” which they called “Harry Hunting.” This involved the group chasing Harry with the intention of catching and beating him up. However, Harry was very fast and was rarely caught by the gang. Malcolm is mentioned by name only in the context of this gang before Harry's eleventh birthday. He is not explicitly stated to be present during the Dursleys' trip to the zoo for Dudley's birthday, and his activities following Harry's departure for the wizarding world are unknown.
The books provide no physical description of Malcolm. Based on his willing participation in “Harry Hunting” and his association with a known bully like Dudley Dursley, Malcolm is shown to be mean-spirited and a follower. He took pleasure in intimidating and physically threatening his cousin's son as part of a group, demonstrating a cruel nature typical of the other members of Dudley's gang.
As a Muggle, Malcolm possesses no magical abilities or skills.
The name Malcolm is of Scottish Gaelic origin, derived from Máel Coluim, which means “devotee of Saint Columba.” It is unlikely this has any deeper significance to the character, who plays a very minor role in the story.