He is a purple fish with a mohawk and a gray beard. He wears green sunglasses and a blue jacket with two badges, a nametag, two pockets on his chest, one pocket on his sleeve, rolled up sleeves, a folded collar, and a white T-shirt underneath his blue jacket.
When he teaches, he is strict and enforces the rules in the Academy he teaches for. He wants all of his cadets to listen to him and obey his rules. When he is not teaching, he is seen to be friendly and nice.
In the 20th Century, he worked at a place called "Milk Shake." After an accident with a blender, he was unable to continue to make milkshakes.
Later, he works at a a place called "Milkshake Academy" to train sea creatures to make milkshakes correctly.
Captain Frostymug first appears after he jumps from an airplane and lands in front of 6 cadets (one of them, of course, is SpongeBob). He then addresses everyone that he is there to train them into qualified milkshakers. He singles SpongeBob out, thinking he won't know much since he called an ice cream scooper a "ice cream spatula."
Frostymug fails to see SpongeBob's abilities in the blending room after the latter does not even know which controls to use. After a cadet tells him what to do, SpongeBob then makes a milkshake. Frostymug attempts to taste it but it ends up smashing him in the eye. He then wants SpongeBob to experience what a milkshake feels like by putting him into the Shake Similuator. He takes a step further by setting it to "OBLITERATE" and starts shaking SpongeBob around. However, SpongeBob enjoys the ride and wants another go, much to Frostymug's dismay.
He is now angry, saying he has never had such an "unteachable moron" and asks the other cadets on tips on how to teach SpongeBob. One of the cadets actually says that Frostymug is better off "teaching a hunk of coral." He then takes SpongeBob through a series of tests including cherry shooting, blending milkshakes again, and even dropping ice cream from airplanes, all to no success.
He then hosts the Milkshake License Ceremony and gives all the cadets their milkshake licenses, except to SpongeBob. The latter wants to know why and Frostymug replies by telling SpongeBob that he couldn't even get past step one of making a milkshake: filling the glass with ice cream. Frostymug then attempts to show SpongeBob how to properly make a milkshake but starts to get tense as he has a flashback of his younger self serving milkshakes at a place called "Milk Shake." When blending the ice cream, some of it spilled onto an electrical wire, causing ice cream to splash in all of the customers' faces, after which they all scream and run around. Back to the older Frostymug, he then gets his hand caught in the blender and his whole body starts to get sucked in.
Fortunately for him, SpongeBob saves him just in time. Frostymug then tells him that the true secret to making milkshakes doesn't come from machines but from within, to which, SpongeBob says he doesn't get it at all.
Frostymug appears for the final time at the Krusty Krab on a wheelchair and gives SpongeBob his milkshake license. SpongeBob replies with saying that if only getting his boating license would be this easy.
Milkshake maker for Milk Shake - Formerly, in the mid-20th Century
Milkshake-making instructor - When SpongeBob visited he had been doing the job for 30 years.
Though his name is "Captain Frostymug," his nametag actually says "Captain Frosty."
According to the storyboard, he was originally going to have a red beard instead of a gray one.
In the Croatian dub, his name is "Kapetan Crnobradi," which translates to "Captain Blackbeard."