Brig Gen Tlas, said to be in his mid-40s, is a member of Syria's most powerful Sunni family, which has given its support to the Assad's Alawite clan for decades.
The general's father, Mustafa Tlas, was Syria's longest serving defence minister, who helped to ensure Mr Assad's succession to the presidency in 2000 following the death of his father.
As a young man, Gen Tlas also attended military training with President Assad.
So, naturally, he was a member of Mr Assad's so-called inner circle, serving as commander of an elite Republican Guard unit.
The general - together with his wife - was also seen as a leading figure on the Damascus social scene, entertaining foreign envoys, artists and the media.
But sources say he has become increasingly frustrated in recent months over the violent crackdown by the security forces on protesters.
He was reportedly under a form of home arrest since May 2011, following his meeting with the opposition to try to start a political dialogue and also because of his opposition to the clampdown.
The current regime, he once reportedly said, was taking the country to hell.
Most of Syria's leaders are Alawite, but Gen Tlas is a Sunni Muslim, and his desertion may encourage other Sunni officers to consider their allegiances.

Fifty-one Syrian refugees arrived in Belfast last month under a government scheme, having been deemed as being at "real risk" in their home country.
Employment and Learning Minister Stephen Farry said language classes would help people settling in Northern Ireland to "integrate into society".
The lessons are expected to cost about Â£20,000 each year.
A second group of refugees is expected to arrive in Northern Ireland before April, and they will be settled in Londonderry.
The lessons will be provided at further education colleges, but would not apply to economic migrants.
Mr Farry said it was in the "public interest" to help refugees settling in Northern Ireland.
"The measures ensure that no one who comes to Northern Ireland seeking refuge from persecution or war is disadvantaged simply because of their immigration status," he added.
He said the lessons would be offered to all people in Northern Ireland with refugee status.
"Analysis indicates that demand for provision is relatively small so any additional costs will be affordable within the department's budget."

Surrey Police, who were alerted to the closure at about 17:30 GMT, said the tunnel was expected to be closed for "at least another five hours".
Officers have been deployed to help with traffic control and co-ordinate road closures and diversions.
The tunnel diverts traffic on the A3 London to Portsmouth road at the Devil's Punch Bowl.
A Surrey Police spokeswoman urged motorists to avoid the area if possible. A diversion is in place.
The engineers behind the Â£371m tunnel, which opened in 2011, said at 1.25 miles (1.8km), it was the "longest under land" road tunnel in the UK.
Its construction followed decades of severe congestion on the A3.
Manaf Tlas, a Syrian general, has deserted the regime of President Bashar al-Assad and left the country gripped by continuing unrest.
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Refugees in Northern Ireland will be offered free English lessons, a Stormont minister has said.
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The Hindhead Tunnel has been closed in both directions owing to what police believe is a "technical fault".