Hilbert and Hilda are the protagonists in Pokémon Black and White Versions while their replacements as protagonists in Pokémon Black and White Versions 2 are Nate and Rosa. So is there anything that has happened to Hilbert and Hilda in Pokémon Black and White Versions 2?
What I think happened to the main protagainst from Pokemon Black and White Versions is that since the characters mother thinks your them when you enter the house and makes a comment that they went out looking for N. Maybe they could have gone to another region since there first thought was N flew away. Also maybe they could be stuck in the rubble of Challengers Cave on Route 9. (Just an opinon don't get mad)
I think they went on to another region, undercover. Or explored the world by themselves - while your character desired to become like them, and become a hero, too. Not sure! But it sucks that they weren't included.
She's basically right there. Hilbert/Hilda apparently left for another region searching for N, and this is probably why Hilbert/Hilda's mother mistook Nate/Rosa for her son/daughter when you arrive at the house in B2/W2.
It's sad they didn't include Hilbert/Hilda in the game, a nice cameo would have been great or even a battle against them like Red in G/S/C/HG/SS.
Pretty much what others said.
They left off searching for N who, like a true troll, returned back to Isshu to save new protagonist from mad Ghetis.
BW protagonists will probably give up at some point, return home where N will wait and say "Where have you been?"