Francis is one of YouTube's most popular characters. Even after 5 years, many people think he's real. Just this week, BroBible ran this 3-year-old video without realizing it was comedy:

But Francis is not, in fact, real. He is a character played by Boogie2988, one of YouTube's most successful homegrown talents, with two and a half million subscribers. To call him "the biggest" nerd/star/whatever on YouTube is a cheap joke, of course, but Boogie has never been shy about embracing his weight (and more importantly, his struggles with it) for his channel:

He has many types of videos, but for the first few years of his channel, Francis was definitely his opus:

Nowadays, in addition to his comedy videos, Boogie is also a prolific and popular commentator on video games, movies, issues around weight, and the news and culture of the Internet:

Sources: boogie2988