It's unknown if Wallak is a real person, was a real person, or what he even is. To most people his existence is simply a joke similar to the phrase "a wizard did it" back on Earth when explaining a plot device that is unexplainable otherwise. In this case an individual named Wallak is often to blame for something someone doesn't understand or to be used as an excuse. It can be used for both positive or negative happenings, so there's no common consensus if he's good or evil, but most examples does leave him with a prankster aura about him. Lost a sock in the drier? Wallak stole it. Couldn't finish your homework in time? Wallak showed up and delayed you.
It's possible to say that this is the first galaxy-wide meme. A lot of races around the galaxy who are appreciative of humor use this phrase when appropriate, while the more serious or gullible ones might be confused about who this Wallak person truly is and take it literally.
What most don't realize is that there's a grain of truth to the story. Someone named Wallak does exist, and he does stick his nose into a lot of places, even if he doesn't quite do everything people claim him responsible for. Any information about his actual activities and whereabouts is sketchy at best.