Read all about the knights of the Empire in the Knightly Orders Guide.
I'm sticking to Battlescribe. Its free and the deluxe version 2.99/year. I use the free version on my phone and desktop computer. As far as I can tell the deluxe version just lets you support the app monetarily and allows you to rename units (Like renaming "Space Marine Captain" to "Captain Spoopypants" if you desire to inject your own fluff).