Fifteen years later, Alexander is the WBC 140-pound (63 1/2-kilogram) world champion.
And Barker, like so many of Alexander's childhood friends, is dead.
Of the 30 young boys who joined Alexander day after day at that gym in the basement of the old police station, at least eight are gone forever. Another 10 are in prison or have spent time behind bars - including Alexander's own brother.
The beauty of sports is in its power to inspire, with life-changing tales of triumph over adversity. Boxing didn't simply alter Alexander's life, though. It might very well have saved it.
"I think about that every day, why did God choose me? Why am I it?" Alexander said, standing a stone's throw from a makeshift memorial to one of his old friends. "Most of the people that grow up in the 'hood don't believe they can make it. They just think that's all there is to life.
"And I'm a living witness that that's not true."
Hyde Park in North St. Louis is the type of place that fosters despair, not dreams. Hundreds of once-proud rowhouses have been abandoned, the boards that cover their doors and windows an admission that no one will be calling them home anytime soon. Vacant lots abound and businesses are few and far between. Money is scarce, opportunities even more so.
Alexander recognized early on that he didn't have many of the things most kids take for granted.
"I remember me standing by our complex one time, I walked out and there was a guy laying dead right there, by the side of my house," Alexander said. "I remember seeing that and I was like, 'Man."'
And while he liked school, and was a good student, Alexander couldn't tell you what he dreamed of being when he grew up. Doctor? Lawyer? Police officer? Kids in his situation couldn't afford to think that far ahead.
"I didn't really think to be anything until I got older and I was around Kevin, who was teaching me that there was much more to life than that," he said.
Kevin is Kevin Cunningham, now his trainer. But back in the early '90s, when the war between the Bloods and Crips gangs was at its height, Cunningham was a St. Louis police officer.
Patrolling the worst of St. Louis' neighborhoods, Cunningham saw up close the toll gangs and drugs were taking. "Seeing these kids being murdered because of the color of the shirt they're wearing, because of the color of hat they had on, it got sickening, you know?" he said.
As he rode through the dirty streets, Cunningham thought of what kept him out of trouble when he was younger. He boxed and played American football before going into the Army, one season leading right into the other.
So he started a boxing program, setting up a gym in the basement of the old police station. Within three weeks, 30 kids were waiting for him at the gym each afternoon.
Few had ever seen a boxing glove before, but it hardly mattered.
"My thing was trying to instill some discipline in these kids where they could go finish high school, maybe some kids would go to college, trade school or something, to where, at the end of the day, you're going to be a productive citizen," Cunningham said.
Alexander showed a knack for boxing early, but he wasn't the most naturally gifted of the group. Barker, for one, was better when they started. So, too, was Willie Ross, who won a Silver Gloves title alongside Alexander in what was Alexander's first national tournament.
Like Barker, Ross is now dead, killed in a September 2008 shooting around the corner from the old gym.
"They were like brothers to me," Alexander said.
In one case, it was his own brother in trouble.
Vaughn, 14 months older than Devon, was a promising boxer himself, 5-0 as a professional, fighting in Las Vegas and Madison Square Garden. The two boys dreamed of how boxing was going to change not only their lives, but those of everyone in their family. But Vaughn fell in with the wrong crowd.
Now he's serving an 18-year sentence for, among other things, robbery and attempted robbery.
"Devon got it the first time," Vaughn Alexander said. "He never took his eyes off that prize. He never took no wrong turns. He didn't want to make no fast money. He didn't want to do those things to jeopardize that No. 1 goal."
Boxing is primal and brutal, not a sport one simply "does" like basketball or tennis. A boxer willingly absorbs dozens of blows each fight, to say nothing of the hundreds of hours of sparring and intense cardiovascular training it takes to get ready for a bout.
It takes a discipline and commitment few people have, and those who are serious are quickly separated from those who aren't.
For Alexander, it was never a choice. He loved boxing from the first day Cunningham put the gloves on him, grinning and laughing when he heard that drumlike "pop-puh-puh-puh-pop" of leather on leather.
So when the other boys cut their training runs short, stopping as soon as Cunningham was out of sight, Alexander kept going. When everyone was urging him to "just hang out," he kept walking.
And when his friends realized the drugs that were already all around them could be an easy means to those new clothes, sneakers and cars that every teenager wants, Alexander spent even more time at the gym.
"I don't want to sound too perfect, but I just never wanted to do anything like that," he said. "I played basketball sometimes, but either I was worrying about school or I was worrying about boxing."
England's Junior Witter, Alexander's opponent in the Aug. 1 fight for the WBC junior welterweight title, is an unorthodox fighter, yet Alexander handled him easily. He dominated for most of the fight until Witter retired after the eighth round, handing Alexander the title.
The American let loose with a guttural scream after the fight and then broke down, overcome by everything he's accomplished - and endured.
It was equally emotional for Cunningham.
"All those years of driving the highways, going to tournaments, staying in hotels, doing this, doing that, you're not thinking about a world title," Cunningham said. "You're just thinking about helping the kids."
Cunningham left the old police station back in 2000, setting up a gym in a rec center on the south side of the city. And Alexander now lives in a quiet suburb, a 20-mile (30-kilometer) move that may as well have been 2,000 (3,000).
But memories of the neighborhood and the boys who didn't make it out are never far away, trailing after him like ghosts.
Tucked in his massive scrapbook, between photos and stories about the many triumphs of "Alexander the Great," are programs from Ross' and Barker's funerals. Posters from some of Alexander's amateur fights still hang in the gym, his young face alongside those of boys who are now dead or in prison.
"I'm the only one standing now. It saddens me to see that, but it motivates me at the same time," Alexander said. "God said to whom much is given, much is asked. I know I've got to do a lot, because I'm the only one here."
