Mister Pück was a doll given to Aelita as a Christmas gift by her parents, Anthea and Waldo Schaeffer (formally known in the show as Franz Hopper). It played an important role as a key to Aelita's past and the overall mystery revolving around Lyoko and Project Carthage, especially in the episode Mister Pück, when it was revealed to hold a key to the locker holding Franz Hopper's diary.
When Jeremie called it a "gnome" in front of Aelita, she got defensive of it (which is only natural because it is Aelita's favorite childhood toy). At the end of the episode Mister Pück, it shows Aelita sleeping with Mister Pück dreaming about her childhood with him. In Mister Pück, he was voiced by Barbara Weber-Scaff.
Prior to finding it, she had frequent nightmares of Mister Puck being chased by wolves, taking refuge in a tree, and finding something. According to Aelita, the dreams are symbolic. Mister Puck finds a small treasure chest with the same visuals as Franz Hopper's diary, and the wolves that Aelita hates the most are all black, likely referring to the men in black.
- The Pück part of Mister Pück's name might be a reference to the character from Shakespearean legend.
- Its outfit's design bears resemblance to William's season 3 Lyoko form.
- Aelita's Lyoko avatar has pointed ears like the doll.