You just got released from the Butterfly Mansion and your crow/sparrow comes back with orders from headquarters

Crow/Sparrow: ORDERS FROM HEADQUARTERS YOU ARE TO MEET WITH MASTER KAYAGA FOR A IMPORTANT MEETING

You make your way to headquarters to see what this meeting is about.

Master Kayaga: I have heard about your growth and development from your crow and others and I would like for you to become to a hashira

You agree and then Master Kayaga introduced you to the hashira