The Diary is a thick pink notebook that Ringo Oginome tried to follow religiously in order to make her "Destiny" come true. It previously belonged to her older sister Momoka Oginome, and after she died, the diary was given to Ringo Oginome in order to bring Keiju Tabuki and Ringo together.
After a diary entry "comes true," Ringo leaves a peach shaped stamp with the word "Destiny" on it.
The diary is believed to be the much sought after Penguin Drum.
How Momoka had aquired the diary or how she knew the spell to transfer fates and why is unknown. After getting the diary, she had used its power to change the world when she thought something was unfair or wrong. An example being so that the school rabbit would become healthy, when it was actually about to die. After meeting Yuri, she used it so that her father would stop abusing her. This resulted in Momoka having a nonlife-threatening, but damaging spontanious combustion. What other things that she had used the diary for is unknown.
The terrorist attack that killed Momoka left nothing of her to be found, except for the diary itself, which seemed completely unharmed. This hints that perhaps it possibly has a life of its own, or it was protected by its own magic. At Momoka's funeral, Ringo was held in their mother's arms along with the diary.
For years to come Ringo would try to use the diary in order to "become Momoka." She believed that the way to bring her family back together and make them happy was to give them back their lost daughter. This included making Tabuki happy as well, and she believed they were destined to be together so long as she followed the diary's written words.