>>55382792The point of Brendan's (and May's) design is to convey that he's a sportsman, the game has a somewhat subtle sports theme with NPCs constantly referencing some kind of tri-athlon that's supposedly going on and there being way more focus on cycling activities.
OG Brendan just wears a full on tracksuit which, even if it's not appropriate for the weather, is appropriate for what his character is meant to showcase.
Emerald Brendan gets rid of the tracksuit so they needed to come up with a more elegant way to convey the sports theme - he wears shorts on top of his bike pants, which is something super common for cyclists.
Without that detail Emerald Brendan just looks like a normal Pokemon trainer.
ORAS Brendan foregoes the double pants but he wears tight bike shorts instead which convey the same idea.