Personality of antagonist is also interesting to think about. For me personally, I put more efforts in planning the personalities on the antagonist more than that on the protagonist.
That is because the existence of antagonist affects greatly to the storyline, and it also give the greatest influence to the protagonist. So I must be careful on coming up what kind of personalities the antagonist get, such that it will lead the storyline to go smoothly.
Personally, it seems that I often goes to the route of
Dark is not evil and
Light is not good. I often choose what people normally think it "look badass" to be the supporting characters that helps the protagonists, and those that look neutral and/or celestial to be the villains.
In the fic I'm currently working in, the storyline can basically divide into three parts, where the three parts each have a central villain/antagonist. For the first part, Porygon Z was the main antagonist. I do not wanted to call it villain, because it is not absolutely evil, and it will become a supporting character in later stage. To tell the truth, this Porygon Z does not have a personality, its brain was broken for a certain reason, so it act like a robot for that whole first story part until the protagonist defeat it and fix it. But for me as a writer, this kind of cybernetic behaviour may also counted as one of the "personality". It was quite interesting when I have to think like a computer during my plot planning.
In the third part, Arceus as the main villain, and oppositely in here Giratina was a supporting character that helps the protagonist.
Stereotypically speaking, I know most people would said this characterization is strange. Arceus was known to be the god of creation, and Giratina was known to be the rebellious Pokemon sealed in another world. But this personality stereotype can really kill one's imagination, so I just jumped out of the box.
Because Arceus was the god, it will get angry if one tries to destroy the world. I made Arceus in my fic to be high-and-mighty, despise the protagonists that try to defend the human which it wanted to defeat.
The Giratina in my fic was not rebellious as like the stereotype, but oppositely well-informed about all the things happened in the world, just merely it has no power to interfere with any of the affairs in the world.
Yeah, basically speaking, I don't follow much on the personality described in the Pokedex entries. Just like we human do, each single Pokemon had different personalities, so even for the same Pokemon, doesn't mean they must have the same or similar personality and/or behaviour.