Rin Tamaki (玉木 リン Tamaki Rin) is Mari Tamaki's younger sister who serves as a school committee member at middle school.
Rin tends to be more responsible than her older sister, often having to wake her up on weekends. This is made easy by having only a curtain separate the sisters' rooms. Rin becomes surprised after seeing one day that Mari finally cleaned up before leaving her room.
Rin is worried, but supportive of Mari's desire to go to Antarctica. She helps Mari with trying to approach their mother to ask for time off school to go, both for the actual trip and the summer training before it. When their mother reveals that she knew, Rin slips away but continues to eavesdrop. Rin's worry is such that she hugs Mari on the day of the latter's departure.
Rin is capable of making flan pudding large enough to fit in a bucket with a longer base radius than her own hand. She serves one for the family dinner on the night before Mari departs, as well as on the day Mari returns.
In episode 5, Rin helps Mari pack for her trip, by leaving some items behind to ensure she stays within her weight limit. She removes a few books and a peeler - this would become ironic later on as Rin uses that peeler to peel a potato, at the same time that Mari was trying to peel one with a knife while on board the Penguin Manju Go.
In episode 11, Rin and her mother appear on video call to Mari during Showa Station's New Year schedule. They see Mari's tanned face except around the eyes, and laugh. Mari had been working in Antarctica without putting a face covering on.
- Rin's surname Tamaki means "gem, jewel" (玉) (tama) and "tree, wood" (木) (ki).