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 mascot of the in-universe anime RaiNet Kakeru, which both Faris NyanNyan and Mayuri Shiina are big fans of. While Mayuri likes to collect Upa merchandise, Faris prefers playing the game Rai-Net Access Battlers based on the anime.

It is later revealed that a metal Upa capsule toy (which is significantly rarer than ordinary plastic Upa capsule toys) 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.