The reason why I think this quote is pointing towards the Black Knight is because 'Unstoppable Destiny' is the name of his theme in Radiant Dawn, and 'One will live and one will die' is one of his quotes in Fire Emblem Heroes.

Unless he's implying that BK won't be an echo, which I somehow highly doubt. But that just completely conflicts with the narrative that there will only be 2 FE echo fighters. This is clearly pointing to the Black Knight from Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn, likely as an echo of. The problem is, is that in his most recent post, he posted this.

The reason why it picked up is because they guessed was going to be a echo, right? And the 'leaker' also said Soren is going to be in the game, and that there are 2 echo fighters from Fire Emblem. Here's my theory about why the NeoGaf leak is fake.