Altair

A mysterious young girl who wears a military uniform-like outfit and can freely travel between worlds. She is the one who brought the other fictional characters into the real world. She claims to do this in order to overthrow their creators and force them to improve the living conditions of their worlds. However, she is actually aiming for the destruction of the real world. She is capable of flying and manipulating dozens of swords through telekinesis at once for both attack and defense.

It is eventually revealed that she is a fan-fiction character from an online video called "Altair: World Étude," created by Souta's friend Setsuna Shimazaki before her suicide.
(Source: Re:Creators Wikia)