@catx3 Yeah, I'm pretty sure it is. I don't know about the trigger though, unless the author is messing with the timeline a bit, it seems like it happened right around when the fight was starting, which would make sense considering that the fight only lasted 10 seconds and Touya was talking to Nowa while running alone in the hallway, so. The trigger must either be Baku winning or something random or unrelated.
The things that make me confident that it's another personality is exactly what you mentioned, that he called him Touya-san, which hasn't happened before, and how Touya reacted to it. Then there's also the part where Touya says he got fooled by him, because Souichi called Nanpou "Referee Nanpou", which implies that it's the usual Leader, but in fact it was just the other personality being quick witted and gambling on the person next to him being a referee to get away. Which is interesting, because it means this personality doesn't share the same memories as Leader (which isn't that uncommon for DID, I think?). Additionally, Leader also mentioned the MPD, which would also hint at him being his usual self, but there's a little sign that was shown which implies he probably just read that and pieced together where he was. I wonder what kind of traumatic event caused it, though. If I'm right, that is. Guess we'll see. How many more chapters for this batch?
Also, I'm assuming the person talking through the phone is the Leader of Ideal, since it would fit with him being the one who "sent his piece faster than anyone else" (Caracal), and only Ideal and Kurama-gumi infiltrated the tower, so since I don't think Kurama is behind this it makes sense for him to be the one doing it; he's also one of the only characters that have been teased but not yet revealed.