Hard to overstate how instrumental A-chan was to making the giant that Hololive is today. I find her Holotalk interview the best place to see it in English, but to sum up if you can't stand early Kiara voice:
After Sora wanted to be a idol/vtuber, A-chan searched for studios with the right equipment, found Cover and they pitched the idea to Yagoo personally. Keep in mind in 2017 most people were using 3D capture setups and for decent quality it was a massive barrier to entry. So it is literally that Sora would never have been Sora if A-chan hadn't existed, and Cover may have never produced any vtubers, and the entire chain of events that led to Hololive wouldn't exist.
And following on from that... some things I've picked up from various streams:
Some of my favorite early Sora content are her horror streams, where Yujin A would serve as the comedic foil to Sora's stoic unreacting face by screaming at every jumpscare. This was core stream content for Sora early on.
During 3D streams and concerts up to 2020 A-chan was the voice talents would hear in their earpieces. In other words she was directing every shoot, including First Fes. By 2021 she was no longer directly involved in working the 3D studio equipment but back in the very early days she did that too.
Some other notes from Holotalk:
A-chan (in 2021) worked as a director, implied to be a content director rather than the business title.
Her main job is event organization, doing the detailed planning and coordination for talent-instigated or self-instigated content.
For example she decided the concept of the Bloom concert and did the detailed planning, things like the setlist (music that was produced specifically for the project), length, live venue choice, etc. I would not be surprised if this was true for most company-wide events back in 2021 and 2020 as Cover was immensely smaller then than it is now.
She also describes herself as a problem solver for the talents, as a side channel from management. Which so many talents have talked about.
Keep in mind that Sora/A-chan, highschool girls on a mission is quite possibly kayfabe.
@The Rrat ranted about it in the past, and made valid points.
But even accounting for that, A-chan was, of course, instrumental. From early tech support over saving Suisei's flatlining career and several other talents that were likewise on the verge of quitting. Even without the probable kayfabe origin story, Yagoo made Hololive, but A-chan (by way of Sora pushing her in that direction) shaped its culture. No A-chan, no Suisei as we know her. Possibly several other graduated members (Roboco, I hear?). A corporate environment much more resembling Nijisanji than the Cover we know.
A-chan, by virtue of her routine appearances on the Hololive channel, represented Hololive more than any other. Shaped Hololive culture from a time when it had two employees, a VR pingpong game and a single VA until it became the behemoth of today.
Filling her shoes isn't just a difficult task. It's plain impossible. Modern Cover is nigh infinitely different from the Cover she joined and helped shape. It's no longer
possible to have the impact she had.
By dint of seniority, by virtue of her close relationships with so many talents, by the sheer weight of her track record (again, Suisei would be but a footnote on par with Mano Aloe without A-chan's intervention, without her habit of maintaining close relationships with the talents...), A-chan's word assuredly mattered. Maybe less nowadays than in 2020, again because Cover has grown so much, but even so, not someone you can just brush aside.
And that's not something you can replace with someone new in the same role.
Nodoka's doing well, of course. It is clear that she is forming her own personal relationships with the talents, and is good at it.
But A-chan's sheer weight, her personal relationships within Cover (the benefits of starting at the company when the CEO could still bring pizza for lunch for everyone) are
gone.
Cover has changed so much since the Christmas miracle. A-chan, like Yagoo himself, like Gen 0, 1 & 2, is an artefact of a bygone time, when everything was smaller, more personable, more intimate.
Cover is growing up. And while I wasn't really aware of vtubing until 2021, and it'd be rich of me to claim that I miss the early Cover I never experienced, losing this bridge to a mythical past, a living reminder that this past happened, is still sad.
A-chan is a part of Cover's soul.
And as fondly remembered and cherished as I'm sure it is, that part is now gone.