Jo, who requests anonymity, says not all part-time girlfriends are selling their bodies. Although she has met a man with a taste for sado-masochism, who paid her HK$2,000 to “beat” him, she insists there was no sex involved. “I won’t overstep my moral bottom line,” she says.

Her clients usually take her to the cinema or meet up for a chat, Jo says. She states clearly on her Instagram account that anything sexual is completely off the menu. Even so, she receives frequent requests for casual hook-ups – “ML [make love], tell me the price” – which she flatly rejects.

“Many people [on Instagram] accuse me of being a prostitute, but I have learned to be patient with them,” she says.

Like many PTGFs, Jo comes from a working-class family. “Being a PTGF is a high-risk job, but you can earn a lot of money in a short period,” she says. “I am compelled to be a PTGF to earn money. I will quit once I make enough.”

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Jo admits to having concerns that first-time customers may try to force themselves on her, given the stigma attached to the PTGF phenomenon, so she chats with them privately before agreeing on a date, to make sure the man is a “good guy”.

She says most of her customers are introverted, or just lonely and want some company. One had recently broken up with his girlfriend and was looking for a substitute companion.

Spending time with customers does not give her the feeling of being in a relationship, she says. “Love should be long-lasting. We don’t talk about materialistic things or money when we are in love.”

One client asked her to be his girlfriend, but she told him straight that a PTGF is merely a commodity and any relationship is ephemeral.

“I advised him to find a loyal girl who will spend time with him for the rest of his life.”

Jo worries about her family and friends finding out what she does, especially her father, who is very traditional. “But I know how to protect myself,” she says.

Celine, 26, a PTGF and tutor at a language school, is also harassed by men asking her for sex, but she always refuses requests for sex.

“Some people just ask directly, but I guide them to think differently, to consider whether they really do desire me sexually,” she says.

She recalls how one customer, bored in his marriage, asked her to be his mistress. “He said he wanted to have some fun with me. I told him, ‘You have a wife already. You can have fun with her, but not other girls’.” She suggested he take his wife on a trip because having an affair was not a solution to his problems.

A talkative woman, Celine enjoys spending time chatting with clients, many of them well-off professionals, who are looking for a dinner companion or meaningful conversation.

Regulars turn to her for counsel, she says. To the younger ones, she is like “a big sister” they can look up to. To more mature clients, she is an equally good companion, able to sympathise with the problems they face in their careers or family lives.

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Celine also broadcasts live chats on Instagram, where she shares her thoughts on topics that interest her, such as the part-time partner phenomenon, why boys like looking for PTGFs, and even her views on education. She describes her live chats as being “like serious lectures”, and insists many men like her approach because they admire smart women.

Being a PTGF has helped Celine to better understand the way men think and behave, she says. It’s also given her a clearer idea of the kind of man she is ultimately looking for. She says it will help her to better understand a future boyfriend or husband.

She has no problem with the social stigma attached to being a PTGF. “People pay to hang out with me. It’s not much different from seeing a social worker. I have nothing to fear about people finding out I’m a PTGF. After all, I’m not selling my body.”

The part-time partner phenomenon works both ways, and part-time boyfriend Brian, 23, is certainly no gigolo. After an unhappy experience last year brought out a masochistic streak in him, he searched the internet for a dominatrix for some excitement. The woman he met took him to a social event that made him realise how poor his social skills were.

Determined to change himself, he opened accounts on social media and became a part-time boyfriend.

“I don’t have a social circle. I want to train myself to meet more people,” Brian says. He adds that he’d always been embarrassed in female company, and still hasn’t quite got the hang of it.

“I differ from other PTBFs in that I have low self-confidence. While other PTBFs highlight their strengths on their self-introduction on Instagram, I emphasise that I’m not a good PTBF. When I think I haven’t performed well, I feel I don’t deserve the customer’s money.”

Brian doesn’t set standard rates; he lets customers decide how much his company is worth to them, “because I don’t want the customer to feel that what they’re getting isn’t up to their expectations. After all, I gain something [social skills] from being a PTBF and the customer gains something [intimacy] too.”

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Although he finds dates awkward, he is learning to take the initiative. “While watching a scary movie, if the girl is frightened, I pat her on the shoulder, or offer to put my arm around her,” he says.

Few women date Brian more than once. They give him positive feedback, but “these are acts of politeness”.

There is one customer who always comes back, though, once or twice a week. They message each other constantly on a daily basis – at the girl’s initiative. Brian says they act like lovers, but he never truly opens his heart.

“I send morning messages to her, but it is not out of affection. It’s just because we have known each other for a long time. It’s a habit.”

Brian insists he will not fall in love with any of his customers. “I don’t have much desire for a relationship anyway,” he says.