If you were casting the part of finance minister in a movie, you might pick someone like Charles Sousa. A broad-shouldered man with a booming voice and greying hair, he looks like the sort of serious, dignified figure whom you would trust with your money.

In person, Ontario's treasurer could also pull off the role of charming uncle. In his corner office in the Frost Building one recent afternoon, he rattles off jokes and stories with ease, at one point breaking into Jean Chrétien's distinctive Shawinigan accent when describing a meeting with the former prime minister.

A few minutes later, he catches himself just before he wanders too far out on a tangent about his undergraduate days at Wilfrid Laurier University, where he had a co-op placement at a hard-partying Calgary oil company: "I probably shouldn't say too much about that," he says with an impish grin.

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But when the talk shifts to Mr. Sousa's first budget, which he presents on Thursday, there is no mistaking that the significance of the fragile balancing act he must pull off is not lost on him.

"An easy answer is, 'Let's just slash and burn.' But what, then, do you sacrifice? You sacrifice the potential of economic growth. And in a sensitive recovery, that can't be," the 54-year-old says. "But we can't do the tax-and-spend routine because we also want to encourage investment, we also want to encourage people to come and feel confident that they can do business in Ontario and compete."

If Mr. Sousa is feeling the pressure, there is good reason. The province is facing a deficit of at least $9.8-billion. The minority Liberals have already picked much of the low-hanging fruit, and Mr. Sousa acknowledges he will have to overhaul programs and make structural reforms to find long-term savings.

But he must also win the backing of the left-wing NDP to avoid an election with his party sagging in the polls. And he cannot alienate the Liberals' big-tent base, which nearly came unstuck last year over a battle with teachers' unions.

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The budget, he says, will show a road map for achieving balance by 2017-18, and reform government to that end. "I want a budget that speaks to an economic plan – not just a fiscal reality," he says. "There are certain things we can harmonize, there are certain programs we can modify."

As the nation's economy sputters, all eyes will be on the previously little-known man shepherding its largest province's finances. Succeed, and Ontario continues on its road to solvency; fail, and the province will be plunged into an election that could sweep its ruling party from office.

In many ways, Mr. Sousa's entire life has prepared him for this.

The businessman

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Born in Kensington Market, then the bustling hub of Toronto's immigrant communities, he grew up in admiration of his father, Antonio, a gregarious Portuguese immigrant who ran a string of businesses – rooming house, restaurant, wholesale food operation. The family relocated to Mississauga, where Mr. Sousa has lived since.

Antonio Sousa says his younger son was a busy kid, playing football and swimming in addition to his studies. "Charles was always a going guy. He never stopped. He was a very good student, a very good kid," he says.

Two books sparked his interest in politics: the memoirs of former finance minister Walter Gordon, and a tome about IT&T. At the time, he says, he was decidedly right-of-centre, believing people should sink or swim based on market forces.

After taking his business degree at Laurier, he set up a small financial-services company. A few years later, he joined the Royal Bank, arranging loans for small businesses. There, he says, he learned the valuable skill of satisfying a need for capital while remaining fiscally prudent.

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"The easy answer is, 'Well, that doesn't work. No. You can't do that, it's too generous a request,' " he says. "But I would say, 'I can do it if we make these modifications … either put up more equity or change the balance sheet.' I always found a way to make it work. And next thing you know, they succeed."

In the early 1990s, he went back to school for his executive MBA. As a final project, his work group toured factories in Mexico, Brazil and Argentina. Mr. Sousa's Portuguese and his ability to make connections came in handy, recalls classmate James Oosterbaan.

His sense of humour showed during a trip to Iguazu Falls, when they piled into a 12-foot Zodiac and got soaking wet near the roaring water. Passing a big boatload of tourists on the way back, Mr. Sousa jumped up and shouted in Spanish: "Oh my God, we barely survived!"

"That almost emptied the larger Zodiac because he looked so bedraggled," Mr. Oosterbaan laughs. "That was the quality of the guy – he was a lot of fun."

Mr. Sousa joined the Federation of Portuguese-Canadian Business and Professionals, becoming president. Among other projects, he helped organize a trade mission to Portugal and set up programs to teach children Portuguese. His ethics were also clear: Once, the federation needed a legal opinion, and a lawyer on the board offered to provide one. Mr. Sousa maintained that would be a conflict of interest. "What impressed me was his ability to get everyone working together. He always took the high road," then-board member Joe Pinto says.

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Through his involvement in the community, he began to rub shoulders with politicians, meeting Paul Martin in the early 1990s. Over the years, as Mr. Martin rose to the top of the federal government, Mr. Sousa was one of the business leaders he consulted.

While Mr. Sousa clearly had a grasp of economics, Mr. Martin told The Globe, he was also interested in policy areas ranging from immigration to labour to health care.

"He understood – and it was very much a part of my own make-up, there was a compatibility of views – that you weren't going to deal with the financial deficit unless you also dealt with the social issues that led to it, like poverty and the inability that some people had to have a real opportunity," Mr. Martin says. "He was a person of ideas, which is one of the reasons that I would seek him out."

Around this time, Mr. Sousa was promoted to the bank's government-relations department.

It was a difficult era. Mr. Martin had recently turned down RBC's attempt to merge with the Bank of Montreal and the country's finance industry was trying to repair its relationship with Ottawa. Mr. Sousa's calm, conciliatory style was an asset in that endeavour.

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"He is a consensus builder. He's a realist. There's not an ounce of ego with him, and he's not a brash 'look at me' guy," says Charles Coffey, his boss at the time.

He made the same impression on then Liberal MP Steve Mahoney.

"He had a sense of stability," Mr. Mahoney says. "I never saw him as an 'ambitious-type politician,' but more of a stable, take-care-of-the-knitting type of guy."

Mr. Sousa credits his time as a lobbyist with developing his understanding of social policy. It helped that his boss was Mr. Coffey, who co-wrote a provincial report on early-years education. Under Mr. Coffey's mentorship, Mr. Sousa says, he developed interests in women's issues, education and aboriginal rights.

As he approached mid-life, Mr. Sousa began to think about jumping into politics. His motivation, he says, was to leave a legacy.

"To feel accomplished, to be able to do something of more value," he says. "I'm looking at: What are my kids going to think of me? How are they going to benefit from the work I do?"

The politician

Mr. Sousa joined the Liberals around 2000, becoming a fixture at party events and making friends in the Mississauga South riding association. One was Darryn McArthur, a business consultant and long-time Liberal volunteer. Another was Tanya Guy, who says people "gravitated" to his modest, personable presence.

"I'm not sure if he had that real passion or that drive or that need to be in politics," she says. "But he really wanted to do something meaningful with his life."

Ms. Guy, Mr. McArthur and a few friends became the nucleus of Mr. Sousa's first campaign team. In 2007, when MPP Tim Peterson defected to the Progressive Conservative Party, Mr. Sousa defeated him in the subsequent election.

The morning after, Mr. McArthur recalls, he and Mr. Sousa went for breakfast at a pub and he caught a glimpse of the new MPP's unflappable demeanour.

"There were people that came up to him and said, 'I didn't vote for you,' and, 'I supported Tim Peterson,' and, 'I never thought you would win.' Charles just smiled and said, 'Well, thanks for voting anyway,' " he laughs.

His stock rose steadily. He was promoted to labour minister in 2010 and moved to citizenship and immigration the next year.

"People saw him as someone they want to work with," says Helen Burstyn, a long-time Liberal insider and adviser to the Ministry of Economic Development. "He makes you feel when he's there like [your] group is the most important thing and the only thing he cares about."

He fostered personal loyalty in volunteers and staff by regularly attending riding association meetings and socializing over beer and wings after.

"He would just talk to us, not like a politician at all, which was very refreshing. Just sit down, roll up his sleeves and he really was able to take off that political hat and just be a regular guy," says Darwin Harasym, who volunteered in Mr. Sousa's riding.

He opposed the construction of power plants in Mississauga and helped negotiate a deal between the province and the city to turn the site of the former Lakeview Generating Station into a mixed-use neighbourhood.

He placed fifth in the race to succeed Dalton McGuinty, throwing his support to Kathleen Wynne after the second ballot at the convention.

Ms. Wynne comes from a different wing of the party than Mr. Sousa – she was a social activist before getting into politics – and is generally regarded as a policy wonk, while Mr. Sousa is more managerial. But multiple sources in government say the two have similar styles. Both are good at listening and taking advice, and try to build consensus before acting.

Even in private, those who have worked with Mr. Sousa say, he never loses his cool, in contrast with most tightly wound politicians. He also takes pains to ensure he or his staff respond to queries and listen to people who want his attention. The habit eats up his time, but has paid off politically: he has a large network of loyal friends and acquaintances.

Some aspects of politics he is still getting used to: he still gets slightly nervous before important speeches.

The qualified yes

In downtime, Mr. Sousa likes to play piano or hang out with his wife, Zenaida, and their three children. He's close with his father and older brother Julio, a school principal. He also likes to unwind over drinks or a meal, particularly at Portuguese restaurants.

And despite his increasing workload, he keeps in touch with everyone from friends to former volunteers.

"As much as he's focused on what he's doing when he's at work, Charles likes to undo the tie and kick back a little bit and just be one of the guys," says long-time friend Joe Eustaquio. "He's a cool guy to be around."

When Ms. Guy was in hospital after her children were born, she recalls, he dropped by unannounced outside visiting hours: "He had on this huge smile and this even bigger bouquet of flowers. I could tell just by looking at him that he had charmed all the nurses."

Mr. Sousa insists his easygoing friendliness will help him deliver in these tough fiscal times.

When fellow cabinet ministers ask for money, he doesn't reject the requests outright, but shows them the province's financial state and asks for suggestions on finding it in the existing budget.

"Instead of saying no, I'm saying … 'This is what we have to deal with. Given these circumstances that are before us, what are you willing to do?' " he says. "And very quickly, they're coming to the same conclusions that I, as a finance minister, have come to. … They're saying 'Alright, we're saying yes under these time frames and under these conditions.' It's not a no, it's a qualified yes."

With a report from Grant Robertson