World Trigger

is a Japanese manga written and illustrated by Daisuke Ashihara. It was first serialized on the Weekly Shōnen Jump magazine starting in issue 11, released February 2013.

Plot
One day, in Mikado City, a gateway to a parallel universe had opened. From it, an outbreak of aliens known as Neighbors wreaked havoc across the city with their own sophisticated technology. Suddenly, a mysterious group of people brought themselves together to repel their attacks. Soon after, Border was formed, an organization researching Neighbor technology and maintaining a defense system against the Neighbors. Its members are armed with weapons known as Triggers. Four years later, the city has recovered from initial Neighbor attacks to the point where its civilians have become used to them.

Yūma Kuga is a transfer student from a foreign country admitted to a local school. There, he meets fellow classmate Osamu Mikumo and a group of bullies harassing the two. After fighting off the bullies, a Neighbour appears. One of the bullies is about to get eaten, but Osamu, actually a member of Border, activates his Trigger, but fails to defeat the Neighbor, leaving Kuga to activate his own Trigger and defeat it successfully. Kuga reveals he isn't actually part of Border, and that his Trigger is a memento of his dead father who wanted him to move to Japan. He also reveals his father wasn't part of Border either, and that he actually transferred from the alternate dimension where the Neighbors come from.

Other Sources

 * Japanese Shonen Jump World Trigger website
 * English Shonen Jump World Trigger website
 * VIZ Shonen Jump World Trigger website