When did it ever confirm that Yen Sid was a brother pupil to Xehanort and Eraqus? I don't recall reading any sources that explicitly site that Yen Sid trained alongside those two.
Sigh. Did I miss something from an interview again or what?
It wasn't confirmed. The only thing that IS confirmed is that Yen Sid is old friends with Eraqus and Xehanort and walked "the road of Keyblade Master" with them for a while.
As far as I can recall, Yen Sid's relationship with Eraqus and Xehanort was never even officially addressed in an interview.
All information we get on that one is from the game BBS itself from statements made by Eraqus and Yen Sid himself:
Eraqus said:That was my dear old friend Yen Sid. As you know, he is Master no more... but he still keeps a close eye on the tides of light and darkness.
Yen Sid said:I had hoped, Xehanort, your heart would no longer lead you astray.
Yen Sid said:There was a time when I walked the path of the Keyblade Master alongside Eraqus and Xehanort. But the three of us have gone our separate ways.
It may be the last sentence in the second Yen Sid-quote (which you get as a text bubble when you talk with him as Terra) which leads some people to the interpretation that they once formed a trio.
This is my bad for derailing the topic, but it was never confirmed it was just a debated point. I always assumed they weren't, and this doesn't really prove anything
Indeed, the question still cannot be answered for good as "old friends" can also include some joint training, even if Yen Sid, Eraqus and Xehanort did not have the same Master.
This issue manifested itself in an interesting way when 2.8 was announced. I got the feeling on some of the comments that they thought that once again Nomura decided to push the release of KHIII with a spin-off. But if you have been following the series closely, you realize that actually 2.8 is a missing piece of the story, that a part of the fandom has been wanting to see for a very long time. The missing part was also considered to be told in III, but now they have a chance to tell that story, and the rest of it in III.
About the Yen Sid topic, in the beginning Eraqus mentions him as an old friend. Also if my memory is not playing tricks on me, I remember that if you talk to Yen Sid in BBS, there is text bubble where he says something about walking the same road as Eraqus and Xehanort, but I am not sure if that confirms them being trained together or anything.
Those who derail 2.8 as a spin-off are most often those who derail any title/entry not having a clear roman numeral as an "unneeded" spin-off and are also seldom worth to waste any time on as you won't be able to sway their preconceived notions.
Those who are truly following the whole series, even if they simply did not have the finanical means to participate in the whole console-spread mess, know what exactly 2.8 is for.
Just like the 1.5 and 2.5 Remaster, this third remaster is the last module to finish the preparation for KH III and unlike the first two Remasters, it also contains entirely new material which even the closely following parts of the fandom do not know.
Yes, I posted those quotes from BBS up above. Even with the character diagram from the Memorial Ultimania though it does not clear up anything for good.