Just the fact that he can't see well because of his blindfold, doesn't mean that Isa can't recognise somebody's voice. That Keyblade master. The one chosen by human half of Xemnas before he was Xemnas. The one with whose Nobody, Xikur, Lea became so close. The one because of whom the memory witch had to disappear.

Isa hated Riku, but he tolerated his right to existence... or something... because well, he decided not to break Lea's heart again for the time being.

Somewhere in the far reaches of his forgotten memory, a small voice was saying that this could be... who? A strange name that he didn't truly know. An imaginary....

"Oh, naturally. That's why unlike me, they won't call you Xikur."

Yes, he knows how much Riku hates the name. Which is exactly why Isa will always call him that.

Of course, he doesn't know that this is another person entirely, who might think him crazy right about now.