Marth, Ike, Lucina, and Robin from the Super Smash Bros. series of amiibo are all compatible with Fire Emblem Fates. The characters can be scanned in the My Castle area of the game. After scanning them, they will appear in your plaza and challenge you to a battle. Speaking to them will produce some humorous fourth-wall dialogue about their time in Super Smash Bros.
Robin has the most straightforward recollection of his time as a fighter in another game, saying “I have the strangest feeling that before coming here, I battled against many combatants… launching people, getting launched…”
When speaking with Ike in the plaza, he tells the player that “I just remembered… before coming here, I matched up against countless warriors from all over the world to hone my skills.” When you enter battle, the camera zooms in on Ike, who performs his iconic taunt from Super Smash Bros.
Continuing the trend, Lucina has much of the same memories that Robin and Ike do. After stating that she’s come here to challenge you, she mentions “that battleground,” a place where she “honed her skills by matching up against competitors from around the world.”
Finally, there’s Marth. He claims that he’s happy to see you again, then continues, saying: “As we spoke before, my bonds with everyone have deepened through battle.” He then claims that he wants to know more about you, and then challenges you to a fight.
After defeating the amiibo fighters in a battle, they will ask to join your party. The main character will mention that s/he’s “is in the middle of an adventure,” to which they will each respond differently. Robin reflects on his own adventure, having fought many powerful enemies and making good friends.
Ike notes that by following you, he’ll become more powerful. Lucina notes that she came to this world to borrow your power, a nod to her role in Fire Emblem: Awakening. Marth notes that he understands you better now, and would like to join you without interrupting your journey.