It should be clear who this charming homeless man is by now. Dennis is, in fact, Dennis Tanner, son of Elsie. He, along with our Ken, appeared in the very first episode. He appears at 2:42. You can tell they aired these shows live back then, otherwise he wouldn’t have been obscured by Pat Phoenix’s head for so long.
You’d think, however, that they would have painted over his name on that windowsill after 51 years.