I'd argue there's a ton of thematic connections between Kieran and The Loyal Three. Munkidori is described as having been dimwitted, Okidogi as weak, and Fezendipity as drab, before having their traits enhanced by poison. Meanwhile Kieran goes from a
very sudden shift from timid to confident and brash. He also stole Ogerpon's mask at one point in the story, and was even the one who ended up somehow waking the Loyal Three up.
It's also worth noting that immidiety after finding out the truth about Ogerpon, Kieran is found right by the booth.
You can even see Pecharunt in this screenshot. It wouldn't have haved to have moved far from its spot to corrupt him.
I mean it could be a coincidence but I dunno, this feels intentional. Especially since this is right where his villain arch begins.
The theory is Pecharunt took advantage of Kieran's envy to manipulate him and corrupt him. But when Kieran manages to overcome his envy, that causes Pecharunt to wake up and take a more active role.
I don't think the theory has to be outright stated if its well implied. Sure we were wrong about the nature if the paradoxes, which I think has everyone feeling a little iffy about the Pecharunt theory too, but...I still think it holds some water.