If there's any sense or justice in the world, BQM will move Charlie in with Babs as soon as possible, and sort of soft-retcon the abandonment at the end of BoP into "I want you to finish out the semester without switching schools, and the new apartment will take a few weeks to make it livable anyway, so stay with Helena until your classes end, then bounce over to Gotham." She doesn't even have to be second-tier supporting cast, she can just periodically be mentioned or show up in the background of a Babs-at-home scene, as long as it's understood she's there.
What will probably actually happen, of course, is she'll show up and get all mad at Babs for replacing her, and hare back off into the aether whenever BQM's done with her like Babs had never taken her off the street in the first place. And there will be headdesking, mightily, yea.