>I don’t know enough about Advent yet, but Biboo’s fanbase is Pebbles

Interestingly, the Pebbles are both Biboo's fanbase and her mascot, with the original pebble she drew on stream being the mascot version. Her fans were then invited to draw their own #pebblesona and submit it on her Twitter, and they occasionally make an appearance in her streams.

As for the others:

Shiori: Mascot- Yorick, the sad-looking blob on her shoulder.
Fanbase- Novelites, little one-eyed knights.

Nerissa: Mascot- none yet, unless you count her family*
Fanbase- Jailbirds, a blue-black puffball bird, similar to KFP (maybe cooked in a different sauce?).

Fuwamoco: Mascot- Peroccino, the puffball dog with wings who appears in their opening. Pero is treated as something of a villain by the twins.
Fanbase- Ruffians, who don't have a design yet.

*Nerissa's family have all had some level of interaction with her streams thus far; this isn't unusual per se (other siblings have shown up regularly, and there have been jokes of having a HoloMoms generation at some point, going so far as to have Kiara's mom on the official Minecraft server, and she's a fairly talented builder).

However, Nerissa's family takes this to a new level, with official character art designed by Nerissa, and Aradia (her elder-but-not-eldest sister) doing karaoke streams with her, and helping to make a cake for her birthday. It helps that Nerissa and Aradia are both trained singers, with Nerissa being more talented, but Aradia's no slouch either.

"This is probably a stupid question, but . . ."