I honestly think that, while those traits contributed greatly, they’re not the most important factor. Explosive growth in Vtubers are usually because that Vtuber filled in a niche of some sort.
Gura was the return of a long-forgotten meme-queen on YouTube, along with being genuinely entertaining (as well as being part of the first English-language agency Vtuber group).
Kobo had shown herself to be very talented in a lot of things (music, web design, gaming), and is a very Indonesian streamer, as shown by the jokes, memes, and references she makes (similar to how Gura and Calli are very American from the jokes and references they make).
Looking at other Vtubers with explosive growth, one can look at Salome (THE fastest growing Vtuber in history, growing to the big One Million in three weeks) and Vox (who recently hit One Million subs himself and is, by far, the most subscribed in Nijisanji EN).
Salome turned heads with showing her endoscopy results on her debut stream,but what got her ball really rolling was her shorter streams and her strict adherence to character at all costs (which makes her breaking character all the funnier).
Vox decided to also stream on BiliBili, which brought in the massive Chinese audience.
