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.
Prior to finding it, she had frequent nightmares of Mister Pück being chased by wolves, taking refuge in a tree, and finding something. According to Aelita, the dreams are symbolic. Mister Pück 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.