I need help with character development

[LaDestitute]

I need help writing a part of my Pokemon rom hack's storyline. So, the rival is like Gen V's Hugh (personality wise), except he becomes more like Gen II's rival gradually. I need a good reason as to why he turns from your best friend to your worst nightmare. The villainous team is like Team Galactic (goal wise, they want to get their hands on a legendary Pokemon that is said to hold all of the absolute answers and truths to the universe...which can be extremely dangerous in the wrong hands) and the hero/heroine isn't really morally ambiguous at all. They won't do anything wrong, and will always be encouraged to do the right thing.

 

[Jumping Jack Flash]

Well pokemon's always been very kiddy so it doesn't have to be much. Maybe the player character beats the rival to gaining a pokemon, or entering a tournament. Or nabs a badge which the rival then goes on to lose, for some reason caused by the player character (the player knocks something over, or messes up their pokeballs).

 

[Byron]

Jumping Jack, I have to disagree with you. You see, pokemon is a childish game at face value. Capture pokemon, beat elite 4, however the story and plot line all come from the bad guys and your rivals.
Heck, even in pokemon red there were some pretty deep subplots. (God I love subplots)
Look at things like pokemon saphire and ruby. In ruby, there were a team of people who wanted to sink the world/Make a volcano erupt. On top of that you had your rivals, May, who was a traveling researcher, helping her dad fill up the pokedex, and stephen who was a frail ill boy that wanted to take a step into the world before he succumbed to his illness. (Maybe not that dramatic.)
There are lots of other examples to chose from, but I'll leave it at that. LaDes, I'll think up some ideas, although my ideas aren't really toned down things for pokemon x'D