People behind the glass
The people behind the glass are those in charge of the mixing desk and of talking into Simon's ear.
The people behind the glass include Robin, Simon (referred to by Mr Mayo as "Robin's assistant", although he is in fact the show's producer, and as "Stealth Ninja" by Mark, as he sneaks coffee into the studio without Mark noticing), Nicolas the French Engineer, Rob the sound engineer and Jamie the assistant producer.
In a comparison with the animated film Inside Out they may in fact be imaginary and are entirely located within the presenters' heads. This being the case Simon is Joy, and Robin is anger and misery.