The guy who cited the example of Mario and his VA for 20+ years is closer to the mark - it doesn't matter how much his voicework contributed to the growth of the character - he doesn't have any ownership of it. You can make the same argument for vtubers and the "talents" behind them - they're all either employees and/or contractors hired by the company to voice/operate the company's IPs
The law doesn't really recognise "an IP", it recognises various specific fixed (i.e. recorded) creative expressions. E.g. the character designer's artwork is a creative expression, which comes with both copyright (very likely owned by the corp) and moral rights (which cannot be sold or transferred). So the corp controls how that artwork is used, but the original designer still gets credited for it.
Voicing generally isn't afforded much protection. But writing is, and livestreaming-era vtubers create a lot of their characters' lore on the fly, and that's something they would definitely own the moral rights to. Someone doing Kizuna Ai style pre-recorded skits using someone else's script wouldn't own anything any more than Martinet does, but with today's streamers it's a lot less clear.
The corpo-friendly precedent would be something like Disney mascot characters, who ad lib live. But Disney is very careful to not record those performances and not allow the performers to contribute anything back to their characters' canon - so while those people may own their own performances, they don't own anything that Disney uses and there's no way to claim that Disney is infringing on their rights. It's quite different for vtubers.
to say they have ownership over their character's name is a step too far, imo. The VTubers may be playing the character, but the companies are the ones that boosted their names through advertising, sponsorships, merchandise, etc.
In principle I agree, but it's a situation that the company has brought on themselves by not properly crediting the talent, so I'm not going to feel particularly bad for them.
Again (and as per another reply) the talent wouldn't own "the character". They'd own that stage name. Things like character designs and song recordings would still be owned by the company. It would probably be impossible to produce any content with that character if the company and the talent could not come to any agreement, but that's already pretty much the case nowadays in practice if not in theory.