By Tom Balmforth, John Irish and Max Hunder

KYIV (Reuters) - Intense. Impatient. Sleep-deprived. Step into the relentless world of Volodymyr Zelenskiy, Ukraine's wartime president.

The 46-year-old said his ambition when he was elected in 2019 had been to help Ukraine become a modern democracy, before that mission was shattered by Russia's invasion in 2022.

"All I wanted five years ago was a very liberal country with a liberal economy," Zelenskiy, a former stand-up comic, told Reuters in an interview in May on the fifth anniversary of his inauguration.

This week, he instead found himself professing his desire to kill Russian President Vladimir Putin as he expressed anger and anguish over an airstrike that hit Ukraine's largest children's hospital.

The war-hardened Zelenskiy who's exhorted Western leaders to action at the NATO summit in Washington in recent days is a world away from the political novice who became president, let alone the TV comedian who was a showbiz heavyweight for years before.

He once even won Ukraine's version of "Dancing with the Stars".

The clean-shaven, boyish Zelenskiy sworn in as president in Kyiv in 2019 wearing a stylish suit fitted to his slight frame has been replaced by a much older looking, heavier-set, brooding figure typically clad in paramilitary fatigues with unshaven stubble and dark circles under his eyes.

Zelenskiy largely veered away from questions about himself in the interview with Reuters, instead focusing on his deep frustrations with some of Ukraine's wartime allies and returning to his central message: the West must to do more to help.

Reuters spoke to eight current and former Ukrainian and foreign officials who have worked with Zelenskiy, as well as several friends and colleagues from his past.

They paint a portrait of a leader who has become tougher and more decisive, less tolerant of mistakes and even prone to paranoia, as he copes with round-the-clock stress and fatigue.

"This is a sleep-deprived regime," said Zelenskiy's former defence minister Oleksii Reznikov, adding that the president was often on the move around Ukraine and had a "grab bag" with a change of clothes and a toothbrush because he frequently didn't know where he'd be spending the night.

"This is the president's daily life - broken sleep. It is consultations at night and addresses to parliaments, senates ... regardless of the time," Reznikov said. "He's in stress mode 24 hours a day, seven days a week - it's a never-ending marathon."

There's little tolerance for the ill-prepared.

Zelenskiy will order officials and advisers out of the room if he feels they're not fully ready, according to a member of his team, who recounted how the president dismissed his aides in frustration during a meeting earlier this year to plan the information campaign surrounding the mobilisation drive.

"If he sees people aren't prepared or are contradicting each other, he'll say, get out of here. I don't have time for this," said the team member who was present at the meeting and requested anonymity to speak freely about Zelenskiy.

Many of the people interviewed spoke of being impressed by Zelenskiy's mental endurance and his ability to cope with his role as Ukraine's president, wartime commander-in-chief and bridge to the world.

"His memory is a huge strength. He keeps a large amount of information in his head, he very quickly grasps details and nuances," Reznikov said. "This gift accelerated his rapid mastery of the English language - I watched it."

Former minister Reznikov, who was dismissed by Zelenskiy in September 2023 after corruption scandals at his ministry that he denied any connection with, dismissed any suggestion that a former TV funnyman with scant geopolitical experience could take on the might of Vladimir Putin's Russia, whose forces overwhelmingly outnumber and outgun Ukraine's.

"I would apply Mark Twain's quote to President Zelenskiy," he added. "It's not the size of the dog in the fight; it's the size of the fight in the dog."

At the same time, Zelenskiy has grown increasingly "paranoid" about suspected Russian attempts to assassinate him and destabilise Ukraine's leadership, according to a senior European official who has held talks with the leader.

"And rightly so," the official added.

PLAYING PIANO WITH HIS...

Zelenskiy's grave appeals to the NATO summit this week present a stark counterpoint to the irreverent comedy sketches that sent audience into howls of laughter in years gone by.

One YouTube clip from 2016 shows Ukraine's future leader standing behind a piano with his trousers around his ankles, "playing" tunes despite his hands being nowhere near the keyboard, to the delight of the crowd.

"Of course he's changed over the past five years," said Andriy Shaykan, who studied with Zelenskiy at the Kryvyi Rih Economic Institute between 1995 and 2000. "He's become older, as a person upon whom an incredible burden is placed. He sleeps for a few hours a night. That huge pressure - it shows."

Zelenskiy grew up in the 1990s in Kryvyi Rih, a steelmaking city in central Ukraine that was consumed by economic turmoil and rampant crime after the breakup of the Soviet Union.

He found his niche in entertainment, building a hit comedy troupe - named Kvartal 95 after his home district - which won the KVN Russian TV talent show popular across the former Soviet region.

In 2015, Zelenskiy starred in a new TV sitcom "Servant of the People", playing an honest school teacher who becomes Ukrainian president after a classroom rant about corruption goes viral online.

The role struck a chord with Ukrainians fed up with post-Soviet graft and, in an extraordinary case of life mimicking art, helped catapult him into the president's office in a landslide vote.

Artem Gagarin, a writer for Kvartal 95, admits he was baffled when his former boss decided to run for office.

"He was Ukraine's top comic, basically the top show-businessman. Why did he need this?"

Five years on, he says he is grateful that Zelenskiy chose the path he did, as he has proved himself a natural leader.

"Otherwise, where would we be now?"

'A MILITARY LEADER'

Zelenskiy certainly isn't universally loved at home.

His public approval rating, which leapt to 90% in 2022 after the invasion as Ukrainians rallied round the flag, has been dragged down by war fatigue, an unpopular conscription drive, the sacking of a respected general and a grim battlefield outlook that has seen Russia slowly advancing in the east in recent months.

A president elected to drain the establishment swamp in a fierce expression of Ukrainian democracy has become ruler of a country under martial law.

Zelenskiy's main political rivals have been frozen out of key decision-making about issues such as military strategy, governance and international relations throughout the war and many ordinary Ukrainians have voiced unease at the concentration of power in his team's hands.

"People now do not perceive him as previously, as an anti-establishment politician, a former comedian," said Anton Hrushetskyi, executive director of the Kyiv-based KIIS pollster. "They see him as a military leader and all the jokes from the past, people leave them in the past."

Zelenskiy's public approval has stabilised at around 60%, which is "high considering the overall difficult situation" of a war that is dragging on with no end in sight, Hrushetskyi added.

U.S. Representative Michael McCaul, the Republican chair of the House Foreign Affairs Committee who has met Zelenskiy several times in Ukraine and in Washington, told Reuters that he had grown into his position as an inspiring wartime leader.

That process began when he refused to be evacuated by the West at the start of the war as Russian troops bore down on Kyiv, McCaul said.

"Zelenskiy is always serious, and gets to the point," he added. "I remember meeting with him and his generals and they gave me a list of weapons that they wanted."

FRUSTRATION WITH ALLIES

Despite having supporters like McCaul and U.S. President Joe Biden, Zelenskiy has struggled to retain global attention for Ukraine's plight since the Israel-Hamas war erupted in October last year.

His persistent appeals for more Western aid are often imbued with a moral indignation that Ukraine is paying in blood to defend the democratic world from Russia.

"He repeats 15 times what he needs, that we need to do more or face the consequences, and he doesn't let it go," said the senior European official.

The Ukrainian leader has become increasingly frustrated with Western nations, according to a second European official who said he would be well advised to "tread carefully" to avoid alienating much-needed allies.

At meetings and phone calls with foreign officials, Zelenskiy hammers home the same message, relentlessly pushing his cause, two European officials told Reuters.

More recently, in a subtle but notable shift of emphasis since a summit in Switzerland held to garner international support and isolate Russia, he has underlined the urgent need for a fair resolution to the war and talked of a second summit later this year that could include a representative from Moscow.

"We don't want to drag out this war and we must reach a just peace as soon as possible," he said in Kyiv after talks with Slovenia's president on June 28.

Trying to ramp up pressure on NATO on his way to its Vilnius summit last year, Zelenskiy lashed out at the military alliance saying it was "absurd" that it failed to give Kyiv a clear timetable for it to join.

In Washington this week, with that goal still elusive, the Ukrainian leadership was less abrasive, with his chief of staff saying he was happy with its outcome.

Zelenskiy himself has warded off questions about how he has performed as leader of Ukraine under exceptional circumstances.

"I cannot assess my activity, I think it is not very ethical," he said in the interview with Reuters at his office in central Kyiv to mark five years in power.

"I am proud that I am the president of Ukraine – this is my attitude to all these five years."

(Reporting by Tom Balmforth and Max Hunder in Kyiv and John Irish in Paris; Additional reporting by Pavel Polityuk, Olena Harmash and Mike Collett-White in Kyiv and Patricia Zengerle in Washington; Editing by Mike Collett-White and Pravin Char)