I had forgotten about him until you mentioned him and I watched the posted video. So, I looked him up. I was surprised to learn who he was and why he was on the show, but it made sense. Below is info from a Google search:
The "Roy" character on The Mickey Mouse Club was Roy Williams, a beloved Disney animator and artist personally chosen by Walt Disney to be the "Big Mooseketeer," a large, funny, relatable adult figure who connected with the kids and designed the iconic Mouseketeer ears, serving as a warm, skilled artist and gag man amidst the young Mouseketeers.
Why Roy Williams Was ChosenWalt's Vision: Walt Disney saw Roy's large, distinctive physical presence and humor as perfect for an adult Mouseketeer role, telling him, "Say, you're kinda fat and funny looking. I'm going to put you on the show, and call you the Big Mooseketeer!".
Animator & Artist: Roy was a talented Disney animator known for quick sketching, designing the famous mouse ear hats (inspired by a 1929 cartoon), and creating many visual gags for the show.
Kid-Friendly Personality: While some adults found him difficult, the child Mouseketeers adored him for his warmth, sense of humor, and willingness to help and draw for them, making him a trusted figure.
His Role on the ShowThe "Big Mooseketeer": He was the large, friendly adult presence alongside head Mouseketeer Jimmie Dodd, providing adult perspective and humor.
Creator of the Ears: Williams designed the famous Mickey Mouse ear hats, a core part of the Mouseketeer identity.
Talented & Witty: He was integral to the show's spirit, known for his quick sketches and practical jokes, making him a cherished part of the club.