Ritsu Koide was a member of the basketball team and one of the victims who was dead during the disaster days after the bus accident. Though her parents are doctors, she doesn't know anything about the medical treatment. She is a timid girl, often deferring to Chiaki Sakashita.
Grisaia no Kajitsu Edit
Immediately after the bus crash, Captain Sakashita assigns her to care for the wounded, based on the fact that her parents are doctors, though Kazuki ends up doing most of the work, with Ritsu assisting.
Afterwards, Sakashita puts her in charge of the food. On Day 6, a carrot ends up going missing, and Kazuki uses a trick to disguise the theft, to prevent people throwing accusations at one another.
On the night of the 15th day, she helps chase down Amane and Kazuki. She and Saaya Kaneda manage to catch and restrain Kazuki
Grisaia no Rakuen Edit
Koide and Kaneda restrain Kazuki, until a fight breaks out, resulting in the deaths of Minori Sakuma and Haruna Ibuki, and with Chiaki Sakashita sustaining severe wounds. Following this, Ritsu ends up working as an assistant to Kazuki, caring for Sakashita and later Kaneda. After Captain Sakashita reveal the location of the emergency ration to Kazuki, she accompany Kazuki and Kaneda to retrieve it. As the fight break out between Kazuki and Kaneda over the food, she threathen to throw the food away before Kaneda collapse from the pain in her head.
Against Kazuki's warning, she makes herself sick when she gorges herself on the rice from the emergency ration, causing her to vomit uncontrollably, and rip the lining of her stomach. She contracts Hepatitis E from eating meat from Tamaki Hirooka, but ultimately dies from hemorrhaging in her stomach when she steals some rice, gorges herself on it again, and begins vomiting uncontrollably again, dying in that spot.