>>56061498Generic kodomo hotheaded dummy with a heart of gold and boundless curiosity. It'd become standard because it works, and it worked especially well with the way early Pokemon was written.
Over time he was watered down into basically a battle autist who was very excited to introduce the new pokemon of the episode. Everything else was sanded away with the excuse of him having "grown up" over the course of the series. The end result being a vapid shell of a solidly realized character archetype that it was fun to root for. But that's the price you pay for never switching MCs for decades.