Kingdom hearts 3 ideas

[Sephiroth]

Or just like Cloud, Tifa, Sephiroth, Yuffie, Yuna, Rikku, Paine, Setzer, Cid, Vivi, Zack, or Seifer, they'll call him exactly what they called him in the game..

You have basically confirmed what I said because he is called Nanaki ingame and only Red mentions his subject name once. Cloud, Tifa, Sephiroth, et cetera have no special nicknames - except for fans. Nanaki is known as both Nanaki and Red XIII. And Leon was also very fitting for Squall because of his whole I have my own symbol and that is Griever story.

I don't say he will be called that. I just mention reasons why decisions are normally made and why the assumption is logical. Actually I doubt he will appear at all.