United States Olympic boxer Joseph Diaz, Jr., at the Team USA Media Summit in Dallas, Texas, on Tuesday, May 14, 2012. (Toni L. Sandys/The Washington Post)

Joseph Diaz Jr. is small, just 5 feet 5 and 123 pounds at 19. A decade ago, he was smaller still. And in South El Monte, Calif., small means vulnerable, and vulnerable kids get bullied. Before he was a teenager, he would come home, harassed, and hide the bruises. His parents would wonder: How is he doing?

“Dad,” he would say, “everything’s fine.”

He and his parents went to a baseball game and sat in front of a kid who wore one of those giant foam fingers. Instantly, Joseph Jr. — known as “Jo-Jo” — found himself on the wrong end of the finger. He was poked. He was prodded.

“What the heck’s going on here?” his father, Joseph Sr., remembers thinking. “I don’t even know how to explain it. It just seemed like my son kind of attracted that kind of attention.”

The attention Diaz Jr. will receive at this summer’s London Olympics has more to do with being the harasser than the harassed. Diaz’s path to becoming an Olympic boxer is almost cliché: a picked-on kid who went to the gym more as a means of defending himself, and found a potential career.

“I want to win a gold medal,” Diaz Jr. said. “And then I want to turn pro, and be a world champion.”

Long ago, Diaz left behind his origins in his sport — simple self-defense — but his motivations remain as primal. Joseph Diaz Jr., a trucker by trade, is out of work. His wife, Valerie, lost her full-time job and is working part-time. Diaz now sees the Olympics as a path to prosperity, not just for himself, but for his family.

“I just feel motivated,” Diaz Jr. said. “I feel that if I just help out my family, that just gives me that extra boost whenever I’m in training.”

Joseph Diaz Sr. raised his three daughters and his son in South El Monte, a working-class town of some 20,000 wedged into the intersection of Interstates 10 and 605, east of Los Angeles, where the median household income is roughly $40,000. The city’s Web site boasts that it has “one great characteristic that separates them from their neighbors: OPPORTUNITY.”

Joseph Diaz Sr. hoped that opportunity for Jo-Jo would be baseball. He played when he was 6. He played into his teens. Baseball. Baseball would be the route out.

“I could probably get a scholarship,” Jo-Jo said. “But I’m so small. I might not be able to play professional baseball because I’m so small.”

The baseball diamond could be a place for bullying, too, and the kids on the streets of South El Monte don’t take kindly to those who look like prey. Joseph Diaz Sr. had always wanted his son to steer clear of such problems. “I don’t want to see my son fighting,” he would tell Jo-Jo. But he also sensed his boy would be a lifelong victim if he didn’t defend himself.

“I’m kind of giving him mixed messages, I guess,” Joseph Sr. said. “He was like, ‘What do you want me to do? You’re telling me not to fight; and now you’re telling me to fight.’”

At the South El Monte Community Center, a local man named Ben Lira ran a boxing program. Joseph Sr. didn’t know anything about boxing, other than a vague notion about what it might provide his son: a chance to stick up for himself.

“Son, how about the gym?” he asked.

“Yeah, Dad,” Jo-Jo replied. “They’re picking on me.”

The first day Diaz Jr. walked through the doors, an older, bigger boy — one of those who had bullied him in the past — challenged him to a sparring session. Diaz Jr. begged for some time, just a week. He wanted to at least learn how to lace up gloves, how to throw a punch. A week later, he took on his adversary. This was nine years ago, and when Diaz Jr. talks about it now, it’s like it was last week. His eyes widen.

“Gave him a bloody nose!” he said. “I made him cry. And ever since then, I just got excited. I was determined. I told my dad, ‘You know what? I’m focused on making this my dream.’”

It became, then, his father’s dream, too. Lira provided the guidance, but Diaz Sr. dove in headfirst. He read books on boxing. He looked for training techniques and guidance on YouTube.

“I was able to pick up enough,” he said. “But it’s not really what I’ve shown Joseph. He’s always been really athletic. He has some God-given abilities and talents. But you know, the kid works his butt off. He’s always put in that little extra to compete. That’s what I believe has gotten him to this point, to where he’s an elite amateur boxer.”

He will be that for the next two months, regardless of how he does in London. If it had been up to Diaz Jr., though, he would have turned pro two years ago. His parents convinced him to give the Olympics a shot.

“I’m not anxious for him to turn professional,” Diaz Sr. said. “He wants to turn pro. He knows if he turns professional, he can make money. He knows I’ve been out of work. It’s been hard here. It’s what he wants.”

What he wants now: a medal, then a contract.

“I know I can do it,” he said. “I know I can keep getting better and be a world champion.”