I really like the Guardian as a character and I subscribe to everything that you guys pointed out in this thread.
During my first playthrough I decided to trust the Guardian, especially after the events of the crèche and the conversation with Voss who mistakenly thinks I am referring to Orpheus when I am talking about the person in the prism. Obviously at that point we are unaware of Orpheus and Voss is unaware of the Emperor and that is actually a clever way to turn a misunderstanding into an engaging mystery.
After the guardian is revealed as the Emperor I was even more intrigued and really enjoyed following the quest in which you investigate Stelmane murder scene and then find his personal belongings and the meeting room of the Knights of the Shield. In hindsight I feel like that part should've been expanded more, because you really don't get much of a sense of what the Knights were actually doing and they don't feel much distincted or different than the Guild... just another vague organization pulling the strings from the shadows.
In reality, their leaders are devil-worshippers in the service of Gargauth (we only get a VERY VAGUE HINT of this in one of the books we can find in the room with the round table where we fight the Githyanki which references 'moral decay' if I am not misremembering!) which makes it super weird for the Emperor to single-handedly taking over such an organization (and makes me wonder if this has something to do with Gortash's suspiciously lacking involvement in the plot). It also makes you wonder if Stelmane was really an innocent victim or if she was up to something more sinister. As many people have pointed out, the Emperor's behaviour is very incosistent and changes completely depending on our attitude towards him. If you're nice to him and pass a few checks before he is revealed to be Illithid, he flat out tells you that the last person who was kind to him was Stelmane and at that point in the story you have no idea who she even is so it's just... odd. If he told you that to manipulate you into trusting him, it would make no sense for him to bring up the specific name of someone that as far as he know is still alive and has dirt on him. Is just weird, and makes me inclined to believe that, just like with Ansur, he actually considered her a 'friend' and thought that mind-controlling her was 'for her own good' or found some other way to justify it in his imperscrutabile Illithid way of thinking.
There's much more tha could be said about his character tbh, but I'm going to stop here
