Upa

Upas are a well merchandised character within the Steins;Gate universe, first seen as a dispenser prize, then later seen as a pillow. Upa is the very popular (according to Rintaro Okabe even more popular than Pikachu) mascot of the in-universe media franchise RaiNet Kakeru. Mayuri Shiina is especially a fan of Upas due to their endearing design. Most Upas are in plastic, which makes the Metal Upas one of the rarest and most valuable among the Upas. Exclusive Upa plushies are also handed out to participants competing in official RaiNet AccessBattlers tournaments.

Steins;Gate
It is later revealed that a Metal Upa capsule toy is actually an integral part of causing World War III in the Beta Attractor Field. At the time of Dr. Nakabachi's press conference, Mayuri drops the Metal Upa she originally wins from a machine in the Radio Kaikan building. Kurisu Makise subsequently picks it up and puts it with her time travel thesis. After Dr. Nakabachi steals Kurisu's time travel thesis, the Metal Upa sets off metal detectors, resulting in Nakabachi taking it in his carry-on luggage when he defects to Russia. This prevents the thesis from being destroyed by a fire in the luggage compartment.

If this Metal Upa is replaced with a plastic Upa, Kurisu’s time travel thesis burns up in flames in the luggage compartment after Dr. Nakabachi steals it and flees to Russia, thus preventing World War III from happening. Preventing Kurisu's death is also necessary to prevent World War III and achieve the Steins Gate Worldline.

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On the Beta Attractor Field Mayuri Shiina would eventually buy a Fairy Upa that was featured in a recent movie. She would keep this Upa and later give it to Kagari Shiina as something to remember her by, who would then take it with her to the past. Due to the years having taken a toll on the toy it has become degraded. On some Beta worldlines, Kagari loses it in the Future Gadget Lab and subsequently tries to retrieve it in which she suceeds after a short battle with Suzuha.