Abel being a friend of Robin's family means there's a nonzero chance that he'd know Katida as well (making it possible that Jun would know her too); as far as the AU goes, Kat and Robin are cousins with their moms being sisters, and actually Robin's staying with Kat's family for the time being.

Unlike Robin, though, Kat's been living in Kaisou for her entire life (Kat's parents moved a good while back, Robin's are still back home), but she's a closet nerd who hasn't gotten out much because she doesn't like dealing with most people offline (Abel and Jun would be exceptions, ofc).