The serial killer known as the Lonely One is a major character even though we never directly see him. Or do we? Nobody knows for sure whether or not he's the one whose body they haul out of Lavinia Nebbs's house. Anyway, despite the sketchiness of his character—both literally and figuratively—the Lonely One has no small job in Dandelion Wine: he represents death itself. The night Doug goes missing in the ravine and his mother worries he's dead, she says:
"The Lonely One's around again. Killing people. No one's safe anymore. You never know when the Lonely One'll turn up or where." (10.41)
Note that Doug's mom doesn't give any detail about what the Lonely One actually looks like, or any indication of how she found out about him. In fact, the first time she mentions his name, you wonder if he's an actual person or just an urban (or, in this case, rural) legend. It seems at first like "the Lonely One" might just be another name for the boogey man.
In "Just This Side of Byzantium," Bradbury's introduction to Dandelion Wine, he writes:
Was there a Lonely One? There was, and that was his name. And he moved around at night in my home town [Waukegan, Illinois, on which Green Town is based] when I was six years old and he frightened everyone and was never captured.
Actually however, that's not true—the August 2, 1928 edition of the Chicago Tribune reports that a "cat burglar" named Orvel Weyant was very much captured. Weyant had a strange M.O.: he would break into a business, rob the joint, then write three letters. The first he left behind for the owner, apologizing for taking the money; the second he sent to the local newspaper, detailing exactly how he committed the crime; the third went to the cops, making fun of them for not catching him. Each letter bore the signature—you guessed it—"The Lonely One."
But he wasn't (as far as we know) a murderer, and a serial killer who's never caught makes a way better antagonist for a boy who fears death than a guy who knocks over a few convenience stores and gets busted. The former is much more effective if you want to talk about the universal human terror of dying since it amps up all the most terrifying elements, particularly the fear of the unknown.
If there really was a serial killer going around town knocking people off, you'd think the neighbors would be relieved when he died, right? But in Chapter 31, Charlie Woodman's bummed about the loss of the Green Town boogey man:
"What's we going to talk about now? It's no use talking the Lonely One if he ain't even alive! It's not scary anymore." (31.3)
Luckily, we have the ever-present (and strangely pragmatic) imagination of Tom Spaulding to keep the legend alive. As long as there's an ice house, says Tom, there will be a spook to haunt it:
"I'm going back to Summer's Ice House and sit at the door and pretend he's alive and get cold all up and down my spine […] you got to take your chills where you can find them, Charlie." (31.6)
Before the day is over, the boys have decided that the dead man they saw coming out of Lavinia's house wasn't the Lonely One after all. How could he have been, when he just looked like a plain old random dude? In this moment we see how legends are born, and we can imagine that the kids in Waukeegan are still telling Lonely One stories. Or maybe they've moved on to Slender Man now, like everyone else.
Either way, this is how cat burglars go on to become homicidal maniacs in slumber party tales for years to come. And the Lonely One is how Bradbury makes good and sure that death is ever-present as our main man Doug grapples with the stakes of being alive.