For most of the time VShojo's been a thing they've had Nyanners, Veibae, and Ironmouse as their big 3 most popular streamers. But Zen is a fucking machine at streaming and through the combined powers of a very consistent schedule and bonkers streaming hours has taken the number 4 spot, and is probably in the top 10 vtubers on all of Twitch even with the TTS debuff. To put into perspective how fucking much Zen streams, both Mouse and Froot had subathons this year, each lasting more than a month. The red line in their stats is their total hours streamed, and you can see the huge spike Mouse had in Feb-March, and the spike Froot got in Oct-Nov. Zen has no spike, and streamed more hours than Froot did, and reached 75% of the hours Mouse did, when she's already an extremely active streamer on top of her subathon.
Anyway people like Zen because the experience is a fairly unique one. There's other chats out there that bully their streamers like Chibidoki and Onigiri, but both Zen and chat fighting on TTS feels like both parties are on even footing. Zen also makes extremely good use of a 3D model with over-the-top reactions to compensate for not being able to emote via voice so it's not particularly hard to get past the TTS debuff.
Also the TTS having captions makes it extremely easy to make Zen reaction images since you don't have to get people opening the image up to add text. It's already there. Anyway this has been your informative lesson on why people like Zen.
