I can't say much on how realistic this is, but it's a cool idea so it would be fun to see it work.
Google definitely has the best voice recognition technology out there, including good understanding of dialects compared to Alexa and Siri, so if I were to pick one of the three, I'd definitely go with Google.
Still, I imagine you're going to have to structure it much like an actual voice assistant with all its limitations. You're probably not going to be able to pull off conversations more advanced than what the companies working on these technologies are already doing themselves.
You might have to structure the story around this, perhaps making the main character a robot or something to make the non-human responses feel less jarring.