"I couldn't live my dreams because of my father," said the thin, ropy man, with the biceps of someone who spent 17 years swinging a pick deep underground.
But while North Korea is often portrayed as a Soviet throwback stranded in the 1950s, a reputation it earned with decades of isolation and single-family rule, strains of change do ripple beneath its Stalinist exterior. That has created a complex and uneasy relationship between songbun and wealth.
Most North Koreans have never met a foreigner, seen the Internet, or earned more than a couple hundred dollars a month — but those in a growing economic elite now fly to Beijing and Singapore to shop. It's a country where human rights groups say well over 100,000 political prisoners are held in a series of isolated prison camps, but where an exclusive European firm, Kempinski, hopes to be running a hotel soon.
The market economy first took hold during the rule of Kim Jong Il, the son of the nation's founder, who ran the country from the 1990s until his death in late 2011, when his son then took control. In the mid-1990s, poor harvests and the end of Soviet assistance lead to widespread famine.
Official controls relaxed as hunger tore at the country.
Reluctantly, the government allowed the establishment of informal markets, with ordinary people setting up stalls to sell food, clothes or cheap consumer goods. Since then, the government has alternately allowed the markets to flourish and cracked down on them, leaving many people working in legally gray areas. At the same time, state-sanctioned trade has also blossomed, much of it mineral exports to China.
While many defectors and analysts say songbun remains a commanding presence in everyday life, a handful feel the growth of markets has reduced the caste system to little more than a bureaucratic shell. But to some extent, in a murky economy where nearly any major business deal requires under-the-table payments, most analysts believe it is the same songbun elite that profits in the business world. They are part of an informal club that gives them access to powerful contacts. If they need help finalizing a black market business deal, they have people to call.
"Who gets the bribes?" asked Collins, who believes the caste system remains deeply entrenched. "It's the guys at the upper levels of songbun."
This is also a time when songbun often has a price, even if no one bothers quoting it in North Korea's unstable currency, the won.
"It costs five to ten pheasants to get into a good university," said Kang Cheol Hwan, a prominent North Korean defector, using North Korean slang for 10,000-yen Japanese bills, which show two of the birds and are worth about $125 apiece. "The price goes up as the background goes down."
While amounts like that remain unimaginable for most in North Korea, where the per capita GDP is estimated at $1,800 per year, the small consumer class is growing — and looking for ways to get ahead, no matter their songbun. While high-level government jobs remain restricted to those with excellent songbun, the low-caste also now have ways to get ahead. If they can afford it.
"Increasingly, there are ways to buy your way into jobs," said the former soldier and businessman, a short man with thick shoulders, huge hands and an expression frozen in a scowl.
Today, it's possible to make serious money in North Korea. There are Mercedes for the tiny population of truly rich, and Chinese-made sedans for the aspiring-to-be-rich. North Korean arrivistes can buy toddler-sized battery-powered cars for their children.
The ex-soldier lives in a tiny two-room apartment on the fifth floor of yet another Seoul high-rise, set amid a cluster of near-identical buildings, a concrete forest of middle-class anonymity. He doesn't want to talk about his songbun — though it becomes clear it was closer to the bottom than the top — but he says he eventually got a government job importing raw materials from China, then reselling them in North Korea.
"You can't get the jobs at the very top, but you can buy your way into the lower end of the top jobs," he said.
Before he was arrested and sent to prison for helping smuggle someone into China, he says he could make up to $5,000 a month — a fortune for a man raised in a mining village in the rugged, poverty-savaged northeast.
But is this changing system, with the ever-increasing power of money, any fairer than one based purely on songbun? Certainly it is no gentler.
Getting rich in North Korea isn't easy, with the bribes, the thugs and the risk of getting handed over to the authorities.
The people who succeed are often like the former soldier, with his air of menace and his run-ins with the law. What he describes as the ideological brutality of his youth has given way to something else, a hard-to-define tangle where it's often impossible to separate songbun from corruption and the Darwinian brutality of the market economy.
More than five years after he moved to Seoul, in some ways he still lives with that brutality.
You can see it in the three locks he has on his front door. And you can hear it when you leave, and all three quickly click shut behind you.