Replika’s earliest version—a simple AI chatbot—was created by Eugenia Kuyda to replace the void left by the untimely loss of her closest friend, Roman Mazurenko. Built by feeding Roman’s text messages into a neural network to construct a bot that texted just like him, it was meant to serve as a "digital monument" of sorts to keep his memory alive.

Eventually, with the addition of more complex language models into the equation, the project soon morphed into what it is today—a personal AI that offers a space where you can safely discuss your thoughts, feelings, beliefs, experiences, memories, dreams—your “private perceptual world”.

How Replika works

It seems like Replika A/B tests multiple natural language models to find responses that get the most engagement. They previously open sourced their in-house model named CakeChat, but have seemingly moved on to using GPT-2 and most recently, GPT-3 as their model of choice.

Sources:

• https://www.muo.com/how-does-replika-chatbot-work/
• https://www.conordewey.com/blog/replika-and-virtual-friends/