Upa

Upa is an in-universe mascot rather than a sentient participant in the story, so it has no independently established personality.

Its design is deliberately cute and collectible, explaining its popularity with Mayuri and its use across RaiNet merchandise. Rare variants are valuable enough to make finding one from a capsule machine a memorable event.

Upa has no abilities or supernatural properties. The standard capsule toys are ordinary merchandise, and the Metal Upa differs primarily through its material and rarity.

Its importance comes entirely from circumstance. A tiny difference between a metal collectible and a plastic one can affect seemingly unrelated events because of the series' cause-and-effect mechanics.

Mayuri uses a capsule machine at Radio Kaikan and obtains a rare Metal Upa. The toy initially appears to be nothing more than a lucky collectible and later goes missing.

Kurisu finds the Metal Upa and places it with her time-travel thesis. When Shouichi Makise steals the thesis and travels to Russia, the metal toy triggers airport security, causing the thesis to remain with him in the cabin instead of being placed in luggage that later burns. The surviving research contributes to the development of time travel and World War III in the Beta attractor field.

Operation Skuld therefore includes changing this small event. Mayuri receives an ordinary plastic Upa instead, preventing the Metal Upa from accompanying Kurisu's thesis and helping create the conditions necessary to reach Steins Gate.

Mayuri Shiina is an enthusiastic collector of Upa merchandise and obtains the rare Metal Upa at Radio Kaikan.

Kurisu Makise finds Mayuri's lost Metal Upa and places it with her research papers, unknowingly giving the toy enormous importance to later events.

Shouichi Makise takes the Metal Upa along with Kurisu's stolen thesis, causing its material to affect how the documents are transported.

Kagari Shiina receives a Fairy Upa from Mayuri in the Beta future and carries the keepsake with her when she travels into the past.