Ninja Girl & Samurai Master: Episode 0
Nobunaga no Shinobi: Nobunaga to Minarai Shinobi · 信長の忍び 第0話
Synopsis
Oda Nobunaga saves a girl from drowning one day. She clumsily tells him that she's a shinobi. Instead of attempting to kill him, she swears to serve him one day after hearing his plans to unify Japan. (Source: Crunchyroll)
Source: MANGA