For a time it was thought that there might be a female Cortana avatar (inspired by the game). However Microsoft (probably wisely) chose not to do that.
Cortana aims to go beyond both Siri and Google Now by being a more comprehensive way to interact with Microsoft devices. It entirely replaces the Bing search button on Windows phones and is powered by Bing and all its back-end capabilities. Users can input queries or questions by voice or through the keyboard (which Siri does not).
I'm not at the developer event and so am only reacting to the announcement and some of the details trickling out. From what I can tell however Cortana combines most of the capabilities of Apple's Siri, Google Voice Search and Google Now.
Previously I asked, will Cortana be a breakthrough or a "me too" product? There doesn't appear (from a distance) to be a "wow" breakthough capability that would immediately differentiate Cortana/Windows Phones and tips the scales in favor of Microsoft. However Cortana might impress with subtle or refined capabilities and functionality. There's a lot going on here.
After I've had a chance to use Cortana I'll be able to render a better judgment about its competitiveness and utility. Basically Microsoft had to offer an assistant on Windows Phones if it hoped to remain competitive with Apple and Google.
Cortana will launch on Windows Phones with 8.1 software in the US. It will expand to other non-US markets later.