This sounds a whole lot like what Litchi was trying to do in Chronophantasma by trying to make herself Arakune's observer in hopes that by observing him as Roy that Roy would come back and Arakune would cease, but, that's impossible, for two reasons in the game
1. Kokonoe was already Arakune's observer and that's why he lasted in that state for so long and 2. Litchi doesn't have enough potential to be an observer whatever that meant, it's hammered in even more in Central Fiction where despite Litchi being powerful enough to be considered a "entitled" and as such the right to challenge Izanami for the azure she really wasn't able to keep it because her existence was already fading.
In short, form with how Mori wrote things or how I remember them to be an observer you need
A strong will to recognize not only another indiviual existence but also your own naturally, while rather stubborn, Litchi's will isn't exactly very strong so she alredy field test number 1
second, you need some degreee of ridiculous bestowed on you by either the amaterasu unit, Takagamahara or just so smart you can do it your self like Kokonoe or Relius and we know Litchi isn't nowhere near as smart as them nor is she a special existence like Rachel, Amane and Noel. Her plans were doomed from the start.
So, if our dear main character avatar has the ability not only to observe people enough that he can bring them into this reality, this possibility with their powers and personalities in tack, at least for combat, then this person must really be someone special, maybe even a