The "den mother" of the Shinsengumi, Ayumu (most often known as "Ayu-nee", or "big sister Ayu") is the cook and housekeeper at the Shinsengumi compound. A pretty woman with a motherly nature, Ayumu is fully cognizant of what the people around her do for a living, and acts as a sort of big-sister figure to Tetsu.
She is Susumu's older sister, and, like him, a shinobi. She mentions in one episode them as being apart of the Oniwaban. While Susumu is based on the actual spy, Ayumu is a fictional character.