I noticed those characters the first time I went through the castle in White, but didn't pay a lot of attention. I just noted that they were unique character designs instead of just grunts. The second time through, on a solo run of White, I paid more attention, then later looked them up online.
I've seen no official details on them, but I would assume they were either meant to have more of a role in the first games and ended up just filling in a bit of backstory, or started out as character designs without a set purpose and were included in the games just because the devs liked the designs. In any case, they don't seem to have as much of a role in the games as they could've had.
In BW2, I thought they were actually pretty obviously included - standing right in the middle of the house in Driftveil. I assumed they were included because they have a fandom, and that their lack of any significant role in the story is sort of moot - it was simply, in the non-provocative sense of the term, fanservice. I guess I would've liked to have seen them have more of a role in the second games (and even the first), but I don't have any idea what that role might've been. They're just intriguing enough characters that they caught my attention, so I would've liked to have learned some more about them.