Marth, Robin, Lucina, and Ike can be added to your My Castle feature in Fire Emblem Fates through the use of their Amiibo figures. Tapping them to your bottom screen (or the Amiibo accessory) will load them in and allow you to chat with them. The dialogue is largely unimportant, but sometimes the characters reference themselves appearing in Super Smash Bros.
Upon speaking to them a third time, players have the option to battle that character. Defeating them will add that character to your party for use. My Castle is unlocked in Chapter 6 and becomes more useful in Chapter 7. This means it likely won’t be until Chapter 9 that the Amiibo fighters will join the player’s party. Roy’s presence is not yet confirmed, as his Amiibo hasn’t been released yet. And since Corrin happens to be the player character, well… it’ll probably do something else.