Detractors portray him as a lightweight opportunist ready to say whatever it takes to secure the keys to the Palazzo Chigi, the Italian premier's Renaissance residence in central Rome.
A sharp-suited 31-year-old who whizzes around the capital in an LPG-powered Mini, Di Maio comes across like the ideal son-in-law, a camera-friendly combination of impeccable grooming and manners.
Unmarried, he boasts a glamorous girlfriend, Silvia Virgulti, a party spin doctor ten years his senior.
His style could hardly be further removed from that of Beppe Grillo, the shaggy-haired founder and uncontested boss of M5S, a comic adored and reviled in equal measure for his expletive-rich rants, on stage and off.
When Grillo included a crack about London's Muslim mayor Sadiq Khan “blowing himself up in front of Westminster,” in a 2016 show, it was Di Maio who was trusted with smoothing the gaffe off the news agenda.
“It was a line in a show, it is completely spurious trying to make something more of it,” said the man who will lead M5S into general elections in the first half of 2018. Polls suggest the anti-establishment party could
well emerge as the country's largest.
Di Maio has also distanced himself from Grillo's instinctive Euroscepticism, telling a seminar at the start of this month that: “We do not want a populist, extremist or anti-European Italy.”
The measured tone is a key part of his “ecumenical” appeal to voters across the political spectrum, observers say.
Di Maio was born on July 6TH, 1986 in Avellino, a town west of Naples that is the ancestral home of TV's fictional New Jersey mafia family, the Sopranos.
Although it has its own problems with real-life mobsters, Avellino is relatively prosperous by the standards of southern Italy and Di Maio had a comfortable upbringing.
His father Antonio, an activist for the MSI, a now-defunct neo-fascist party, had his own construction business while his mother Paola was a Latin teacher.
The eldest of three children, Di Maio started out at Naples university as a computer engineering student, later switched to law and never completed a degree.
In a CV posted on the M5S website, Di Maio says he founded his own web and social media marketing business while still a student, as well as working on video projects.
Young and telegenic
In interviews he has also highlighted a spell he had as a steward at Napoli football club before his 2013 election to parliament and subsequent election as deputy speaker in the Chamber of Deputies.
Critics say all that does not add up to much and that Di Maio's primary qualification for high office is his extraordinary capacity for reinvention.
“On ethical issues or on immigration, the thoughts of the M5S candidate are like those of a surfer riding a wave,” commented Catholic weekly Famiglia Cristiana.
Of his own faith, Di Maio says he is “a believer, but not very practising”.
The nature of Di Maio's elevation to the rank of prime ministerial candidate has also come under scrutiny.
With his opponents in the online vote of party members all relatively unknown, many commentators have branded it a coronation rather than an election – with Grillo still pulling the strings from backstage.
“Is it the man who is being chosen or his readiness to take orders and advice,” asked Turin daily La Stampa.
Even among 5-Star loyalists, there are misgivings over Di Maio's inexperience, which one of his opponents in Saturday's vote has voiced publicly.
“He has been imposed on us purely because he is young and telegenic,” said Vincenzo Cicchetti.