Ulrich's and Sissi's Diaries
Ulrich's diary was seen in Log Book after Sissi stole it from him to use as blackmail. Yumi stole Sissi's diary in order for Sissi to tell her where she had hid Ulrich's diary. The entire story revolved around "log books."
Unlike his other friends' diaries, Jeremie's diary is composed of video logs. This was first seen in Temptation when he explained how he can boost the thinking process of his brain artificially using a head device. Jeremie's diary is also seen in X.A.N.A. Awakens and Echoes explaining his adventures as a part of Team Lyoko.
Franz Hopper's diary is a very crucial part of Season 2, as it explains everything he knows about Lyoko, the world that he created. It was first discovered by Jeremie and the gang in Mister Pück when Aelita finds a key in her doll Mister Puck, a key that opens up a train locker containing the diary entries on a large set of CDs. Jeremie's goal was to decode Franz Hopper's diary, which he did in Revelation.