Some characters in the show have (or had) diaries. These characters are Ulrich Stern, Jeremie Belpois, Franz Hopper, Sissi Delmas, and Yumi Ishiyama.

Ulrich 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."

Yumi's Diary

A similar episode to Log Book was Season 4's Lost at Sea. See episode description for details. Only one page is seen, which contains a picture of Ulrich.

Jeremie's Diary

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 headset. Jeremie's diary can also be seen in X.A.N.A. Awakens and Echoes, in which he talks about his adventures as a member of the Lyoko Warriors.

Franz's Diary

Franz Hopper's diary plays a very crucial role in Season 2, as it explains everything he knows about Lyoko, the world that he created and X.A.N.A.. It was first discovered by Jeremie and the gang in Mister Pück when Aelita finds a key in her doll Mister Pück, 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.

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