"Boon didn't actually belong to us" (2.2), says Lucius. In fact, Boon Hogganbeck doesn't really belong to anyone at all. He just sort of emerges out of the woods one day when he's about twelve years old, and everybody likes him so much that he decides to stick around.
When Boon first arrive, General Compson puts him up in De Spain's hunting camp for two weeks during hunting season, and he's the best woodsman in the state—it helps being so tall. During the other fifty weeks, "he had no location" (2.12). Thus, Boon begins living, sleeping, and eating at the Priest's place, since Grandfather employs him for odd jobs every now and again. In no time at all, he's become a regular on the Priest property, recognized for his mad skills with horses and mules.
To Lucius, Boon is family—though he's sort of like My Cousin Vinny, the guy you're embarrassed to have in your family but totally adore at the same time. Boon is a big thirty-year-old kid, but he is well intentioned and kindhearted. He really likes Lucius and sees him as a kid brother.
In turn, Lucius admires Boon for his strength, spontaneity, and playfulness. Check out how Lucius describes him: "He was tough, faithful, brave and completely unreliable; he was six feet four inches tall and weighed two hundred and forty pounds and had the mentality of a child; over a year ago Father had already begun to say that at any moment now I would outgrow him" (2.4). Lucius will certainly outgrow Boon, not so much in height as in maturity. (Seriously. The dude's huge.)
Like a little boy, Boon is fascinated by cars. He loves driving them and tinkering with them to see how they work. Most importantly, he loves Boss Priest's Winton Flyer, and with the owner of the car gone, Boon sees his opportunity to go on a Ferris Bueller-style joy ride. He quickly goes to work, recruiting Lucius to join in his plight.
The funny thing about Boon's plan is that it's not some elaborate plot planned well in advance. Nope: it's a plan that Boon makes up as he goes along. And this is what has Lucius kicking himself later. "All my life," he says, "[he] who dealt with Boon dealt with a child and had not merely to cope with but even anticipate its unpredictable vagaries" (3.39).
In other words, what's about to go down—stealing the car and driving to Memphis—makes no logical sense, but Lucius still should have seen it coming. It's a classic Boon move that's got his name written all over it.
Boon is the catalyst for the adventure that ultimately unfolds. It's because of Boon that Lucius is introduced to everything from the birds and the bees to lying, corruption, and eventually real goodness.
The adventure is far from what Boon could have ever imagined—you know, with the car getting traded for a horse and all—but in just four days, Boon, like Lucius, also begins to discern his own moral compass. He stands up for Miss Corrie after she is blackmailed and subjugated by Butch, and he begins a new life with her when the adventure comes to its close.
Nicely done, Boon.