"Oh, I've got Colonel Fubster managing (the other dogs)... He's retired now, good for him to have something to do..."
-- Aunt Marge (PA2)
Colonel Fubster was a Muggle neighbor of Aunt Marge Dursley. Fubster cared for Marge’s other dogs besides Ripper when she visited Vernon and Petunia on Privet Drive (PA2). Marge had a secret love for him but it remained unrequited due to her “truly horrible personality.” Her longing for Col. Fubster was the reason for her mistreatment of Harry and others (Pm).
Retired from the Military (PA2).
The word "Fubster" is phonetically connected to the word "flub," which means to make a mistake. The "rounded" texture of the word can also give the impression of a stereotypical retired English army officer, probably a bit on the portly side, with a blustery personality, a large mustache and maybe even a monocle.
"Colonel Fubster" also sounds a bit like "Colonel Mustard," a character in the game Clue. That character is also based on the above stereotype.
After Harry accidentally made her wine glass explode with magic when she insulted his mother, Marge mentioned the same thing happening at the home of Colonel Fubster when she was dining with him. This led readers to suppose that perhaps Fubster was a wizard and had also accidentally burst a wine glass with Marge, but the author said that was not the case (BN).