Four years prior to the current News Club was formed, Marin lost her husband to Kanade on his "head clearing" spree after being pissed off about being moved to an area that stunk of fish. Marin found San some time around Season 1 and took her in her Inn as an employee, becoming a mother figure to her since she never had any family. Though she didn't know San's secret about being a Siren due to telling her about her husbands death, which lead to fear of Marin rejecting her. When the News Club visited on their summer trip, a member of Fairy Tale had failed recruiting San and used a disgruntle employee to steal Marin's bankbook. Marin despaired over getting more money, but the News Club came through by calling in customers in their swim suits. She was very thankful to them.
When she went to see to a leftover customer, she found Kamiya waiting for her. Kamiya told Marin San's secret, but Marin was adamant San couldn't be a monster like him. Eventually, Kamiya's song caused San to reveal her true nature and reveal her Siren wings. Marin was shocked that San was a monster as well as the News Club members, leaving her stunned as to what to think. Though she did love San's singing. When San took a hit for her, Marin realized San was still the same girl at heart no matter how much she changed in appearance, calling herself stupid for thinking San would ever hurt her. As she tended to San, Marin watched her recover quickly and take Kamiya apart with her voice.
When San offered her the chance to take her revenge and to take her life as well, Marin only treated her like a silly child like she normally would, proving she still loved her.
After the News Club boarded their bus back to Yōkai, Marin helped San unfurl a large poster telling them to come again.