>Gen 4
Unlike Gen 3, there is no pointer for who is the canon protagonist. I have no idea why people keep saying Dawn is the PC when all they have to rely on is the anime and manga.
The Pokemon that fits each Pokedex holder is based on their design and personality (Chimchar the monkey for the energetic Barry, regal and cute Piplup for Dawn [there's also a corocoro page showing her hugging a Piplup] and that leaves Turtwig for Lucas)
The only way this could happen is if you chose Turtwig as Lucas, meaning that Lucas is the Canon protagonist with Turtwig in Gen 4.
>Gen 5
Who the canon protag is in BW doesn't matter because the other gender isn't essential to the story. Hilda or Hilbert, it doesn't matter.
Fitting Pokemon to rivals, it would make sense for them to be based on personality (Snivy for strategic Cheren, Tepig for bouncy Bianca, and that leaves Oshawott to Hildabert)
For BW2, it's a lot clearer thanks to the promotional trailer. There, it's shown that Nate has a Tepig, Rosa has a Snivy, and Hugh has an Oshawott. This type triangle also shows that Nate is the canon protag.
>Gen 6
Thanks to the Sugimori art posted earlier this thread, Gen 6 is also very clear. Canon protag Selena picks a Fennekin, Calem picks a Froakie, and Shauna is left with Chespin.
There you go. If you got any problems, debate me bae
