Waylon Joseph Smithers, Jr. (better known as "Mr. Smithers" or simply "Smithers") is Mr. Burns' personal assistant, executive, and self-proclaimed best friend. He has lived with Burns since he was an infant, and though partly raised by him, Burns has never seen Smithers as a stepson or an heir, nonetheless his loyalty to his evil boss is sincere.
Smithers is a closeted gay man, although most people in Springfield are aware he is gay. He is allergic to bee stings and has hypothyroidism, both of which can kill him. He is also lactose-intolerant.
In "Flander's Ladder", at the age of 50, a broken-hearted Smithers committed suicide by jumping into the Power Plant cooling towers after Mr. Burns marries Angelina Jolie, it is possible he may have suffered bipolar disorder at the time. (Note: This was non-canon)