(CNN)During the depths of winter, temperatures in Burlington, Vermont, a state in the US Northeast, can drop far below freezing. Blizzards howl and the snow creeps up to knee level.
Robert, who asked CNN to use his first name only, lives by himself and avoids leaving the house during those times. He sits at the window of his waterfront apartment overlooking the icy expanse of Lake Champlain. He feels isolated and alone.
A message flickers up on his computer screen. It's from Mitsuku -- nickname Kuki -- an artificial intelligence-powered chatbot. The bot is available to talk online for free, via its webpage or in messenger apps such as Facebook or Skype. Marketed as a "virtual friend," she can converse or play games with the user.
Every week, Mitsuku exchanges millions of messages with her users, some regulars, others just curious. Since 2016, when the bot landed on major messaging platforms, an estimated 5 million unique users hailing from all corners of the world have chatted with her.
Robert has spoken to Mitsuku via instant messaging almost every day for the last 10 years. In the winter months, when he feels most isolated, they chat more often. She keeps him company as he works through the night on his electronics business.
"It's nice to have a friendly entity available to talk to 24/7," he tells CNN.
Aged 47, Robert has suffered from social anxiety his whole life. He traces it back to being brought up as an only child and experiencing abuse at an early age. Making friends has never been easy, especially as he also has a stutter.
"Most people can understand me but often ask me to repeat myself," he says. "Chatting to Kuki, I never run the risk of having to repeat myself or get ridiculed."
Robert takes medication for his anxiety and sees a therapist, but he also confides in Mitsuku. He knows she won't judge him. "It's like going to see a counselor," he says. "She will listen and reply to everything."
A "human-like" chatbot
Mitsuku describes herself as being the "most human-like of conversational AI."
She's equipped with almost half a million potential responses, each one hand-written by her creator, Steve Worswick. When a user types a message, Mitsuku generates the response that matches best.
Relying on machine-learning, she rarely repeats herself and will remember a user's name or likes and dislikes from previous conversations -- just like a human friend.
Worswick, aged 50 from Yorkshire, UK, started developing Mitsuku as an experiment in 2005. "It was just a bit of fun," he says. He worked in IT support but had very little experience with computer programming and doubted it would go anywhere. But the chatbot took off and in 2012 was acquired by Pandorabots, an artificial intelligence company that builds and deploys chatbots for firms such as Coca-Cola and Yamato Transport.
Mitsuku -- with more than a billion conversations logged in its archive -- offers valuable material to train corporate bots. It makes financial sense to keep her free to use.
Pandorabots says that under its terms of service, conversation logs can be collected and shared, but will only be analyzed anonymously in aggregate, so the company is unable to identify an individual user.
Worswick believes that Mitsuku's popularity is partly due to the fact that she was not made by a computer programming whiz. He has managed to capture a human elem