Mr Susan is the a grumpy old man also known as Rhys Jhenkins. He has no job, is 17 and has a lisp, just like Joshua Mitchell.
He warns Vince of the dangers of vanity, and that if Vince should choose the wrong mirror he'll be doomed to take Mr Susan's place in the Mirror World. However, this prospect that doesn't bother Vince at all, as he can now see his hair from every angle. Vince easily chooses the right mirror, leaving Mr Susan to utter the word 'shit' in annoyance.
Just before Vince has to select a mirror, Mr.Susan says: "My name is Mr.Susan, and now it is time for you to do the choosin'."
The Sandstorm character has been compared to Mr Susan by Julian and Noel in commentaries, as they're almost identical except for the costume material.