The common belief is that Phil groomed Leanna and his ways rubbed off on her as she was just as abrasive and annoying.
I think it was more the other way. Leanna was always the misunderstood snippy starchasing cunt that desperately wanted to escape her bumpkin family she was from her early vlogs when she just started talking to Phil. Leanna is the one who demanded Phil move and have his own place to finally affirm that they were a couple. She's always been the one happy to live in Seattle, and you can see that up to today as she still lives there and has carved out of her own... well, not sussessful, but at least an actual life.
Phil was always a narc idiot, but at least he resembled a human before moving to Seattle. That's what really fucked him up for a multitude of reasons which has been mentioned many times here. He alienated himself with a young girl that was probably a mistake from the start. His Youtube decline was already going, which he admitted at the time, before DUH TROLLS became the eternal boogeyman. He then needed to become a streamer to keep the Youtube windfall going, and we see what that's done to him.
Now Phil is with Katherine, a woman with serious mental issues and ate a house immediately after getting with him.