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Long text but whatever, it's either this or studying.
If we do a bit of psychological analysis based on the snippets of their true personalities they showed on stream. We can infer that Pekora is actually someone who wants everything to go right, plans well and is more of a structured person that we think.
Her road to being the most popular vtuber is primarily due to her talent in comedy but also because she's intelligent and is able to set things up either on the fly or beforehand. You can't just say that it is based on personality alone that she got the top but rather by observing at the games she play and how she interacts with people and build in minecraft, you can infer she is more meticulous and structured than we think.
And it is pretty obvious in the friendship with Miko that who is the one wearing the pants. Miko is clumsier and is more of a follower than a leader. You can easily see that through how she often makes herself the butt of the joke during her interactions but with Pekora she never goes out of her way to do that and often makes the environment or actions be the butt of the joke.
If we assume that this two goes way back and is very close, then based on this we can say that Miko is actually dependent on Pekora and you can even see that through older clips when Miko is sad when she have to take a long break and Pekora reassures her and all of their interactions Pekora actually leads and do set-ups for funny moments to happen between them.
That's not healthy because Pekora IS the more intelligent one and I'm not saying that means she have a better personality. I don't know. But it was always more of an alpha and beta relationship.
I think what actually happened was Miko, with her increase in fame and new interactions with other vtubers, started to see that. And that lead her to act in certain ways or cause an incident that really affected Pekora. I believe Pekora took that as a betrayal because she was always on her side for what the reason might be but one day she decided to take the other side. It might not be an intentional kind of thing but I think, Pekora was really, really close to Miko and trusted her dearly. So in her eyes she probably felt betrayed and abandoned. But she have to wave that off and cope with it as a functioning adult.
Thanks for reading till the end. I won't self plug and say I am taking a psychology degree or what not but my role involves a lot of human interactions but we will never know what really happened and I like to think my POV has an inkling of truth at least.