Cortana for iOS and Android; because everything's connected!
Belfiore writes that upcoming version of Windows 10 will have the phone companion app integrated. If you use Windows phone, the integration will work out of the box. However, if you are an Android or iOS user, you will have to set up right apps on your phone so that Windows 10 can work on your device.
What will the phone companion app do? A lot of things! It will make all the content on your phone available on your PC and vice-versa. For example, if you have OneDrive app installed on your phone, you will be able to sync photos between your Windows 10 machine and phone automatically. It will also let you access all your music on all your machines - a facility for which Microsoft will release a new Music app. If you've stored notes on your phone - they will sync with Microsoft OneNote. You will then also be able to access all your OneNote content as well as docx, xlsx and pptx files right on your phone.

The most important integration will be that of Microsoft's Cortana App. Cortana is a special natural language processing application that searches for information online and lets you do multiple things. It can set reminders for you, tell you if your flight is delayed, remind you to buy milk on the way and set alarm for you. Having the companion app will allow you to access all your Cortana's intelligence on all the devices. For example, you will be able to view all the content on Cortana's Notebook on your iPhone and even complete your tasks you started on Windows 10 on your Android. Belfiore explains it all in the following video -