I think that an objectively good RP character would, as others have mentioned, have strong motivations and traits.
They should be active participants and make decisions that affect the direction of the story.
People tend to get bored if a character is really passive and constantly molded to...
Aspen: Well, I've only got my Vulpix-
*she gestures to it, still following at her heels*
Aspen: -so 'till I catch some more Pokemon, I guess grass would be better. But if the other's closer, I'd be good with that too. Hey, do you have any Pokemon?
*She walks into the store. After a short while, having about everything she needed, she reemerges outside.*
Asper: Thanks for waiting! Good store. Shall we head out?
Aspen: No, of course I don't mind roughing- Wait, you'd take me there? I wouldn't want to get in the way of where you're going- Uh, sorry, where are you headed? I don't think you said any specifics...
Aspen: *she considers the question briefly* Ehh, no where in particular. Well, I mean, if you know where the first gym in this region is...? That'd probably be a good start.
*Aspen walks in Mossbridge town, hoping to buy some supplies. Often disliking solitude, she has her vulpix out and walking beside her. All is quiet and peaceful. Only a few people are out.
Once spotted, she heads to the small store.*
Name: Aspen
Gender: Female
Age: 21
Bio: Aspen did a bit of Pokémon training when she was younger, but is now interested in pursuing it more seriously. She’d love to travel and learn more about Pokémon.
She’s has a Vulpix and is interested in obtaining a firestone for it.
Race: Human