Evans the Striped
Evans the Striped used to be Sports Master at Unseen University. He was in all probability a qualified Wizard, but of a sporting bent, and would have presided over the glory days wistfully remembered by Mustrum Ridcully in Unseen Academicals, when the University genuinely was (at least in Ridcully's memory) a centre of physical prowess and sporting excellence, when UU entered a football team in the city leagues and both Ridcully and the Dean (!) won their Browns for rowing. In A Blink of the Screen, it is revealed that his full name is Gryffydd Tabernacle Evans.
He is remembered as having disappeared abruptly one day when doing press-ups in the Great Hall, and vaporizing into a cloud of dust. As at some point in those forty years, the emphasis of the University changed to a different sort of physical prowess - ie, how physically fat can a Wizard get from competitive pie-eating - it is tempting to suppose that Evans may have been the victim of a magical vendetta from the fat unathletic wizards, whom he would certainly have mocked and derided. The revenge of the fat kids who are always picked last for sports teams, against the PE master who holds them up to the sarcasm and sadism which is virtually mandatory in his profession? A PE teacher can get away with belittling Fatty Muldoon in 2B, but not a fully trained Wizard who, while he is packing the pounds on, is also a sixth or seventh level Mage. And forty years ago, Wizards of course settled such differences with more than just cutting remarks in the Uncommon Room or at a Faculty meeting...
His whistle, however, lives on, where anyone who blows it now carries on his haranguing, making sure that all know that any boy who has not brought his kit will have to "PLAY IN HIS PANTS!!!", which was the terrible fate of every unfortunate whose badly-faked note from his mother didn't fool the PE teacher in England between about 1950 and 1988, when it became illegal under the mental cruelty and educational acts.
"Evans" is also a Llamedosian name. Does this point to a national origin? The very image of Evans bring forth racial memories that make us cringe even now. Whilst he was the wrong nationality, the perfect example on film is Brian Glover's magical-but-terrible Mr Sugden in Kes. Comedian Jack Dee played a similar terrible PE sadist, Douglas Digellby, in seventies' secondary school TV nostalgia-fest The Grimleys. 
and in Terry's own words ()
"......and then I realised that ..mad welsh sports master.. was a tautology..."
The naming of people as Someone the something is a usual Welsh device to distinguish between two people of the same surname, as seen in Ivor the Engine (Jones the Steam) and The Englishman Who Went Up a Hill But Came Down a Mountain (Morgan the Goat, Williams the Petroleum, etc).