Kakei didn't interact with the last Yuuko, but I've always headcanoned that they knew each other rather well in canon and that he probably had tea with her occasionally and gossiped about their respective workers (being Watanuki/Rikuou/Kazahaya). :|b If that's fine with you, to have Kakei know who she is? It's implied in canon that he takes a few jobs/mysteries for her and forwards them to Rikuou and Kazahaya to work out.