It's perfectly possible you've never heard of James Charles.
Birmingham City Council clearly hadn't.
They were rather taken by surprise by the thousands of fans who brought the city centre to a standstill when he appeared for a meet-and-greet at the Bullring in January.
Charles is just one example of an influencer - a social media star who may not be an A-list celebrity by conventional standards, but inspires a cult-like following among fans, which is then often used to build an empire.
Huda Kattan, for example, has been described as "the Bill Gates of beauty influencers" for quitting her job and, with the help of social media, launching her own company, which is now valued at more than $1bn (£760m).
So how should an influencer be defined?
"It's someone who has some influence which they can monetise," explains SJ Nooth-Cooper, a senior manager at Models 1 Talent - an agency which now signs influencers as well as models.
"And they have a USP. So whether it's beauty, fashion, cooking, mental health, being a chef, they can use that to make a business out of it.
"Everyone wants their hobby to be their job... and many people that we've worked with, they've quit their jobs and been able to go 'wow, I'm earning from this one post what I would otherwise have been earning in a month'."
For the generation who grew up studying traditional subjects at school, that can be a difficult concept to get your head around.
Natasha Ndlovu, who has built up 90,000 followers on Instagram with her fashion and beauty posts, says the very term influencer can be a "heated" word.
"I guess that's because there's still that generation, that group in society, that thinks we're a bit obnoxious," Ndlovu says.
But, she acknowledges, it's understandable that many don't comprehend why it has become a profession in its own right.
She recalls the confusion on a postman's face when he began delivering a large number of products which were being sent to her by brands when she first launched her blog several years ago.
"I get a lot of mail. So a lot of the delivery people know who I am, they see me more than they see most people down the street," she explains.
"And so one time, one guy casually said, 'You seem to get a lot of packages', and I said, 'I'm a blogger'. And he was like, 'what's that?'"
Ndlovu then explained that she reviewed products online, but laughs: "I think he was still confused."
Influencers generally make money from endorsing products on their page - which have to be clearly labelled as advertisements because of Advertising Standards Authority rules.
A brand may identify an influencer who has a specific audience they're trying to reach, and pay to appear on their social media feeds in the same way they'd previously have paid for magazine advertisements.
The buzz surrounding influencers has led to major media companies taking them very seriously indeed.
News UK, which owns The Times and The Sun, announced last month it was creating an independent influencer agency.
But while only a few influencers like Kylie Jenner are household names, there are thousands of "micro-influencers" who are able to make a living from being a big name within a specific, often niche, field.
"Micro-influencers are basically the new breed," says Nooth-Cooper, "where you can have from around 10,000 followers to 25,000 followers, and they're kind of the Generation Z of influencers."
Sophie Grace Holmes, an influencer with cystic fibrosis, encourages her followers to lead healthy lifestyles and not to be held back if they have a condition like hers.
Her 34,000 followers might be a few short of Jenner's 131 million, but because Holmes has such a specific audience, brands are keen to collaborate with her.
"For me, I'm really passionate about inspiring and motivating other people," she says.
"My mission in life is to show what you can do when you've been given either the idea that you shouldn't or can't be able to do something."
Generally, Holmes promotes brands and products related to health and fitness - recent endorsements include Holland & Barrett and gym wear brand Beachbody.
But, she says, she will only endorse something she genuinely believes in, and which might benefit her audience in some way.
"If I'm going to work with a brand, I have to make sure that it will aid what I'm doing for my health, my training, fitness, nutrition," she explains.
"My main focus isn't to be paid, it's to have an impact on other people, to push them. I wouldn't just advertise out of wanting to be paid."
Ndlovu has a similarly specific audience, who rely on her opinions on fashion and make-up.
"As a darker-skin woman, when it comes to the high-end beauty brands, people need to know if it's worth investing in an expensive foundation, because it's very difficult to find high street foundations for darker skin tones."
But there are difficulties in becoming known for a particular hobby or cause. Not least, integrity.
How can followers trust someone's taste and recommendations when they're usually being paid to promote particular products?
"Consumers are too savvy and too bored by endorsements and flat product mentions," suggested RocketMill's senior creative strategist Bethanie Mardon in an article for The Drum.
"They are influenced by the influencers they choose to follow, largely because they relate to them and trust them."
But in December, research for BBC Radio 4 showed most consumers mistrust influencers, with 82% saying it's not always clear when someone has been paid to promote a product.
'Be a role model'
"I say no to a lot more stuff than I say yes to," says Nooth-Cooper.
"The way that we manage talent, it's all about the longevity, and the micro-influencer becoming that absolute authority.
"And they have their integrity... they have to be honest about ads and sponsored or gifted posts, but people are following them because they know they wouldn't say yes to everything."
Among Nooth-Cooper's clients are The Bloom Twins, Sonia and Anna, who say they wouldn't put their name to anything they didn't believe in.
"We have so many photos that would have been very successful on Instagram, but they're not us," says Sonia.
"So we're only going to post something that reflects who we are, and that could be us wearing pyjamas and eating ice cream."
All three - Holmes, Ndlovu and The Bloom Twins - make the point that they sometimes keep the more light-hearted or less-polished posts to their Instagram stories, which, unlike images posted in the grid, expire after 24 hours.
Fashion designers have quickly come to appreciate the value that an influencer can bring to their brand.
"Today, you have to ask yourself, what is more important, to have Kim Kardashian in the front row, or [US Vogue editor] Anna Wintour? It's a hard decision to make," Philipp Plein told BBC News last year.
But Anna from The Bloom Twins warns against people becoming influencers solely for the purposes of earning money or becoming famous.
"Don't be an influencer just for the sake of it, be a role model," she advises. "If there's something you want to say, talk about it, so people can relate to it.
"It doesn't matter if you have 20 or 20,000 followers, if you talk about it, it might make a difference to someone's life. If you help someone, if you influence somebody, you're an influencer."