Question

In B2/W2, does it ever mention what happened to the original B/W character and if so where is he/her?

Answer 1

Nobody knows, after Black and White they could have left to travel around the world to finish the pokedex and try to become the number one trainers. Who knows, only Game Freak, maybe we will see them in later games.

Comments

Just saying, that you may also go to your players house in the original, and the old mother will mistake the player for her son/daughter. She will then reveal she know your mother in the sequel, and that she wonders where her son/daugther is.

Answer 2

Well, I know.
When you head on for Nuvema town, the old characters mom says this:

Welcome back dear! Did you find your friend? I think his name was N
N!
N!
OMG! THE ORIGINAL CHARACTERS ARE LOOKING FOR N!
Your Mom then realizes that you are the wrong child, and she talks to you about how she met you Aspertia City mom at a Pokecenter.

Comments

Oh N also makes an appearance in Anville Town in B/W2. A girl loses her Pansage in Anville Town, so you go look for it around Nmbassa City. There will be a green man with Pansage, and they will head to Anville Town. When the girl is reunited with her Pansage, she'll say this:
'Oh silly Pansage. A man with GREEN hair said that Pansage's DREAM was to be a railway conductor'
Who do we know that has Green hair and can talk to Pokemon....?