I'm pretty new to watching vtubers, and the first few I watched were bigger indie Twitch streamers.

I'm now enjoying NijiEN quite a bit but the rigging surprised me. It seems quite stiff compared to the indie vtubers - Bao, Vienna, Shoto, etc. Another example - Frost is indie and not super big, and his model is super smooth and expressive (rigged by Iron Vertex!)

Is it a style preference for nijisanji? Or is there a rule they can only use riggers preferred by the agency? Just curious, I'm still learning a lot about the history of these agencies, idol culture and so on.

(I haven't watched much of Hololive or Nijisanji JP, so that's why my question is specific to nijisanjiEN)