The ‘Phone Companion’ app on the PC will help you install the Cortana app from the Google Play or Apple App Store onto your phone so you’ll be able to take the intelligence of Cortana with you, wherever you go. The Cortana app can do most of the things Cortana does on your PC or on a Windows phone. You can have Cortana remind you to pick up milk the next time you’re at the grocery store, and then your phone will wake up and buzz with the reminder. You’ll be able to track a flight using Cortana on both your phone and your PC, and get the updates on the device that you’re on so you don’t miss anything. Everything in Cortana’s Notebook will show up across all your devices and any changes you make on one device will be reflected when you use Cortana on any of your other devices. The Cortana companion app will help you complete tasks you begin on your PC wherever you are, on your phone.
http://www.techrez.com/microsofts-cortana-for-android-ios-devices/

So you mean we'll still need to download the phone companion app before downloading Cortana?.
What about if you don't have a PC?

Just as BBM's on all mobile platforms...Nice move..Siri next!