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Considering my oshi is Polka and my second favourite is Suisei, I've deduced the three factors:
1. Love for the vtuber industry and motivation to be a part of it (i.e. Polka not giving up after PL, Suisei's journey to hololive)
2. Larger than life personalities (pretty self-explanatory)
3. Genuine friendships (one of the things I love about vtubing are group dynamics, and seeing something like Polka and Flare or Towa and Sui becoming good friends as time passes makes me feel warm inside)