1x23 - My Bride is a Mermaid

Believing Masa was an acquintance of her older brother, Akeno asks Masa about her brother's whereabouts, but Masa had lost his memories before joining the Seto clan. Gōzaburō then reveals to Nagasumi that Akeno's brother is actually Masa himself. While unaware of this fact, Akeno intend to punish her brother's for his irresponsibility, forcing Nagasumi to protect Masa using any methods available.