Like Macron, Edouard Philippe is a pro-European moderate who wants French politics to go beyond the left-right divide.
He is, though, a member of The Republicans party, which poses a bigger threat than any other to Macron's ability to form a legislative majority in June's parliamentary election and avoid a potentially reform-slowing coalition.
The choice of the trained lawyer and mayor of the port city of Le Havre is part of a strategy to undermine the foundations of France's traditional political groupings and transform Macron's own Republic on the Move (REM) party from a year-old political start-up into the dominant force in French politics.
Philippe will provide a counterweight to the Socialist Party legislators who have defected to REM and its 'neither left nor right' cause.
A lawmaker since 2012, his words for Macron have not always been generous. In January, during the presidential campaign, he wrote a column for the left-leaning Liberation newspaper in which he took a swipe at the portrayal of Macron in some media as a French version of former US President John F. Kennedy.
Philippe said Macron lacked Kennedy's charisma and was full of empty promises.
"Macron doesn't take responsibility for anything but promises everything, with the ardour of a youthful conqueror and the cynicism of a seasoned veteran," Philippe wrote.
The following month, he wrote that Macron was "an emblematic representative of the establishment."
By May, however, when Macron was already firm favourite after a first round win, Philippe's words were warmer, and contained advice.
"He'll have to be daring. Get out of the old, comfortable, institutionalised, one-on-one left-right opposition to constitute a majority of a new kind. The path will be narrow, and risky. It's hard to see the 'system' letting go easily."
"A BIT OF A LONER"
Philippe was part of The Republicans' presidential campaign until early this year. He walked out after the party's candidate, Francois Fillon, was engulfed in an investigation into nepotism.
Prior to that, Philippe campaigned with Alain Juppe, Fillon's rival in The Republican primaries and a veteran party moderate.
The son of teachers, Philippe went to school in Germany. Friends say his centrist leanings make a good fit with Macron's vision, and that there is no danger of any ego taking over.
"He's a worker, brilliant, a bit of a loner and discreet, but solid," said one friend who did not want to be named.
Aside from his interest in boxing - he trains three times a week - Philippe has two more things in common with Macron: He passed through the elite ENA school, and his early political hero was Michel Rocard, a Socialist prime minister for whom he campaigned as a student before changing his political stripe.
He was director of public affairs at the now struggling nuclear energy group Areva between 2007 and 2010.
Bearded and balding, Philippe's associates describe a discreet politician.
"This is not a man who is going to tap you straight away on the shoulder, but he is very witty," said Benoist Apparu, who shared the role of spokesman for Juppe's campaign with him.
"He has a real love for the law and judicial issues."
Philippe is also a collector of cufflinks, something style watchers might find harder to keep an eye on than the dandy outfits that former Socialist prime minister Bernard Cazeneuve was known for.