Mamimi Yamagami

Mamimi Yamagami, or Mami (眞深) for short, is initially regarded by those new to the anime as the "13th sister." In fact, like the sisters, she has a pet name for Wataru: An-chan (あんちゃん), or "Bud" in the English dub. However, she is just a girl posing as one of Wataru's sisters after he mistakes her as such, and she is sent by someone to watch over him and break him away from the sisters so she can bring him back to Tokyo.

Mami can be regarded as a pseudo-antagonist because of her mission, but she mostly acts as comic relief, even learning why the sisters love him very much. Eventually, she becomes a source of advice to Wataru and even goes after him when he returns to Tokyo, hoping to return him to the island. In marked contrast to the sisters themselves, Mami treats him differently, acting as if she is more of a friend than a sibling (though towards the ending of the series, however, there are some hints that Mami might be developing deeper feelings towards Wataru). Furthermore, she doesn't bring a lot of things to Promised Island (only clothes and camping gear) with a hand-held computer and possibly a sewing machine as (arguably) her only luxuries.

Although she is not really Wataru's sister, she is somebody's sister. She always receives e-mails from her real brother, demanding whatever happened to Wataru and her plans to bring him back, much to her chagrin. Towards the end of the series, it is confirmed that Akio, Wataru's friend in Tokyo, was Mami's older brother. The two are initially seen to have a bad relationship, with Akio treating his sister more as an agent than as a sibling and her caring little for him. Akio accuses Mami of having grown decadent on Promised Island, whereas she finds his intentions for Wataru to be distasteful. Later, they seem to be growing a little closer to each other, Mami managing to playfully tease Akio and him reminiscing a little about their past together; apparently she used to address her brother with the nickname she has bestowed on Wataru, 'An-chan'.