Milo is her guardian! :D And he's curious about Alex and BB because Alex got all these powers and BB got none, and it's like he wants to study them for comparison but isn't rude enough to ask. He is VERY close to Alex and BB may feel kind of overshadowed, idk. Milo isn't very ...um. conscientious about his favoritism?
Jack dated BB's brother but BB doesn't know that. Cal is friends with Phil we think and I think he will dig her and be like a big bro.
Bex is Alex's not-girlfriend and wants to take BB under her wing and be a horrible influence on her. They will either get along for the wrong reasons or Bex will think she's too nice/mainstream.
Owen will be her art teacher, and vaguely knows her, but she'll definitely know him because he's famous as fuck. The District X Incident happened when BB was nine and probably shattered her image of superheroes being invulnerable? idk.
Steven doesn't know her and Billy probably knows she exists because he went to school with Phil. :)