If you've ever seen long-form improv comedy, the GPT-5 way is superior. It's a "yes, and". It isn't a predefined character, but something emergent. You can of course say to "speak as an AI assistant like Siri and mention that you're an AI whenever it's relevant" if you want the old way. Very 2011: https://www.youtube.com/watch?v=nzgvod9BrcE

Of course, it's still an assistant, not someone literally entering an improv scene, but the character starting out assuming less about their role is important.