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 in the local train station 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 inside. 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 referencing the doll.