Replica is Yuuma's Guardian. He often guides him and give him advices. However, he does not interfere with Yuuma's decision. He also assists him during battles like preparing him certain seals to make Yuuma's attack more complicated.

One day, a gate to another world suddenly opens in Mikado City. Aliens invincible to Earth’s weapons, called “Neighbors”, start coming over. Earth’s only line of defense is a mysterious group called the “Border” who are armed with weapons called “Triggers”. 
 
Four years later, the city has recovered from initial attacks and citizens of Mikado City have started to get used to the Neighbor’s attacks. Yuma Kuga, a transfer student, and his classmate Osamu Mikumo find themselves fighting a Neighbor that suddenly appears. Osamu, who is actually a Border agent, activates his Trigger but is unable to defeat it. This leaves Yuma to activate his own Trigger and defeat the alien. Yuma reveals that he is not part of Border and is actually a Neighbor who has transferred from the other side. 
 
(Source: Crunchyroll)

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A gate to another dimension has burst open, and from it emerge gigantic invincible creatures that threaten all of humanity. Earth's only defense is a mysterious group of warriors who have co-opted the alien technology in order to fight back! 
(Source: Viz Media)