A Brightside is, in fact, an individual who carries a small flashlight in his or her pocket. The flashlight, which is always kept on, is most useful with the Brightside travels at night to hunt its favorite prey, Wendy's raccoons.
The synth band, The Killers, once wrote a popular song about a Mr. Brightside, focusing on a very sad, Emo man who had lost his woman. This "Mr. Brightside" watches as his woman's new man takes off her dress and she, in turn, lets him go. This vision is litterally "killing" the Mr. Brightside, hence the band's name The Killers. After much scrupulation over whether or not a Brightside would take part in such an act, especially without mention of a flashlight, it was determined that The Killers are, in actuality, all a bunch of Brightsides themselves. They also like to hit themselves on the legs with wooden spoons and whip themselves with nettles.