In the Games
Belle is a childhood friend of the protagonist in Pokémon Black and White. Like the player's character and his/her other childhood friend, Cheren, she will receive one of the 3 Starter Pokémon from Professor Araragi. She starts her Pokémon journey the same time as the player despite the protest from her father. She will appear several times throughout the game to help and battle you.
In Pokemon Black 2, and Pokemon White 2. She is with Professor Juniper most of the time, she helps her research new things in the Unova Region. Belle is also the person who gives you (and Hugh) your Starter Pokemon.
In the Anime
She will receive, like the player, one of the 3 starter Pokémon from Professor Araragi at the start of the game, and it totally dependant on the Pokémon chosen by the player. She will pick the Pokémon with the type disadvantage to the player.
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