This is way more prevalent than layfolk are aware of. There are plenty of browser extensions that are basically keylogging anything they say within the browser, and now people talk to AI within their browser.

With decent prompting skills, someone who knows what they're doing has an easy time fabricating realistic looking photographs that are nearly indistinguishable from real life. We're entering a post-truth era.

Honestly, the best way to accomplish this is to use one of the major LLMs and build your own character scaffolding within their toolset. Like, a custom GPT with projects, or a custom Gemini Gem holding knowledge files. You kind of have to build it all yourself unless you want to talk to someone else's pre-configured AI, but it works quite well once set up.