Weren't we talking about Ike this whole time?
Or is it the DLC you're talking about?
So are these other character appearances canonical or what? Can't see Ike and Ragnell suddenly appearing.
You can get special teams on SpotPass in the "Bonus Teams" section. They appear somewhere on the map whenever you make them.
Each have a category (ex. Sacred Stones) from which you can choose a character and some of their army (ex. Ephraim).