Rei Ayanami clones don't just exist in the Neon Genesis Evangelion universe. Since its release in 1995, Evangelion has become one of the most influential anime ever, both in Japan and internationally. Therefore, it's no surprise that one of the series' most popular characters, Rei Ayanami, would inspire a legion of "expies," i.e. exported characters, to the point it spawned a whole new trope.