Theorized timeline of events of Piers’ backstory in Spe
Sonia gets harassed and slandered, so Piers does a bunch of crazy shit to get the attention off of her and onto him instead.
He was more, uh…rambunctious back in the day, as stated by Raihan, and he likely really did attack people.
He successfully gets the bad press, but unfortunately his wild reputation worked too well and has affected his hometown Spikemuth too, further cutting their funding and tourism.
The locals, especially his own Gym trainers, resent him for having chosen Sonia’s wellbeing over theirs.
Piers doesn’t regret helping Sonia but still acknowledges he went too far and f*cked over his own people, so he forces himself to mellow out to try to salvage what he can. There are probably some self-loathing and depression mixed in there. (He will still tear out some throats if you push him.)
(Incidentally, it doesn’t seem he ever told Raihan nor Magnolia about the fallout.)
His Gym trainers no longer trust him to have Spikemuth’s best interests in heart, so when Marnie gets chosen to compete in the Gym circuit, they form Team Yell and do their best to support her, even if they do a backward way of doing it.