I'm actually not entirely sure about that. The stage select screen has an awfully large window saying "William" for it never to change. I'm expecting the primary story to follow William, but I wouldn't be altogether shocked if you could unlock alternate characters for Twilight runs or other challenges (perhaps the Teahouse?).
Confirmations from Team Ninja are hardly the final word on anything. They're about as whimsical as lint in the wind.
At E3 somebody confirmed it would be William only as a playable character. I don't recall who but I know it was mentioned here as well. I suppose I also wouldn't be surprised if there is a way to unlock additional characters but I don't believe they will be playable in the normal story. Perhaps Twilight like you mentioned. Maybe super powerful bonus characters that you don't level like William. It'd be a cool way to get some enticing DLC at any rate. I'd pay to get Ryu as a bonus character.
I don't care about the random YT people. I meant who on the dev team. I'm pretty sure it was one of the producers. Yasuda-san, I think.
I got pretty much everything from interviews, not YT clips. Most of them were posted here by FiendBusa or on Reddit. Don't get salty because I didn't watch the same videos you did.
Why am I getting a Kazumi vibe from this character? She looks very interesting.
